Myth busting: 2-stroke oil in diesel makes it slipperier, right?

By Isaac Bober 26 Min Read

Ultra-low-sulphur diesel lacks lubrication according to the Internet. So, it says, adding 2-stroke oil to a tankful of diesel is the cure-all. Let’s find out.

Ever since it became a thing in the early 1980s, low- and ultra-low-sulphur diesel (ULSD) has been marked as the non-lubricating Public Enemy No 1 of diesel engines. To the point where some, well, lots of people actually, suggest tipping a snifter of 2-stroke oil into your fuel tank every time you fill up to return some lubrication to the diesel.

But this is one of those myths that can only have started because someone half heard the story. So, strap in, take off the tin foil hat and get ready to have your mind blown as the things you thought you knew are completely upended. Because, science.


It’s worth noting that this article applies mainly to modern diesel engines with sophisticated injection and pollution gear. Anyone with something from the 1980s can leave the room.

Ultra-Low-Sulphur Diesel ain’t slippery enough

Trawl the interweb and those mates you’ve never met but sound like they know what they’re talking about will tell you that with less sulphur in ULSD there’s less lubrication and that you need to add stuff to the fuel tank to improve the situation. But it’s at this point where those mates of yours left the room when the talk on ULSD was being given.

See, diesel by its very nature is an oily, slippery substance but high levels of sulphur in fuel causes significant pollution (sulphur dioxide) being pumped out of the tailpipe. So, as countries began clamping down on tailpipe emissions, reducing the amount of sulphur in fuels was seen as a quick way of reducing emissions. Then along came the DPF and EGR treatment to further reduce the emission of fine particles and nitrogen oxides (NOx). Continuing with a high sulphur content in diesel would have killed the pollution gear and all of us too. Eventually.

But, back to the slipperiness of ultra-low-sulphur diesel. Right. The process by which sulphur is reduced in fuel is known as, and take a breath, hydrodesulferization (HDS). This process doesn’t just reduce sulphur down to the current mandated level of 10ppm (parts per million) for ULSD but it also strips away the natural lubricity of diesel (but that’s not sulphur because it only plays a very minor role in providing lubricity to diesel). And this is mainly due to the loss of nitrogen- and oxygen-based polar trace elements. So, you see, sulphur has slight lubricating properties but it’s not the lubricating component of diesel.


Adding slipperiness back in…

Fuel companies, to meet stringent guidelines around diesel composition, must return the lost lubricity to the diesel once the sulphur level has been reduced, and this is usually done via a lubricity improver. And, in the market, there are two types widely used by fuel companies, and they are either neutral or acidic. Both types carry the necessary long hydrocarbon chains with polar end groups (the stuff that’s removed during the reduction of sulphur in diesel). Meaning the ULSD is, more or less, returned to its natural lubricity level.

Is your head hurting? Mine is.

Okay, so the polar head groups in the lubrication additive are what clings to the metal surface; they’re attracted to it, and the hydrocarbon chains are what forms the boundary protection (lubrication of, say, piston rings). Think of it as wrapping the metal bits in a soft blanket to keep them safe.

And it gets more interesting because bio-diesel has been identified as an excellent form of diesel-fuel lubrication – better indeed than anything else on the market. According to researchers at the School of Engineering and Technology, Central Queensland University. “Use of biodiesel with the diesel fuel, especially with the ULSD (ultra-low-sulphur diesel), can serve as a lubricity improver along with the proven qualities of emission and combustion performances one per cent addition of biodiesel can enhance the ULSD’s lubricity by 30 per cent,” the researchers wrote.

So, essentially, the off-the-shelf lubricant additives you can buy is essentially the same stuff the fuel companies already include in their mixture. So, if you want to add one of these commercial lubricants, go right ahead, it certainly won’t hurt your engine…might even help.

Time for a history lesson

Fuel companies have upped their game when it comes to lubricity additives in ULSD. Back in the late 1980s and early 1990s fuel companies were using dimer acid as a replacement lubricant. Why? Because it was being used in jet fuel and it worked very well.


But things didn’t go to plan. The introduction of dimer acid onto the market in Sweden (1991) saw an influx of diesel passenger vehicles suffering from fuel filter blockages, camshaft deposits and fuel pump failures. But why? Well, turns out dimer acids were reacting with the crankcase oils and becoming an insoluble sludge. Dimer acids were subsequently banned from use.

So, just how slippery is diesel?

All diesel sold in Australia, the US, Japan, and Europe must adhere to stringent guidelines on the chemical structure, if you like (for reference the diesel run here and in many other countries is known as EN590). For instance, in Australia (and plenty of other countries around the world) diesel must meet a friction/wear/flat spot ‘measurement’ of 460 microns maximum. So, in some cases, the scar might be less than that if the fuel supplier has added more lubricant.

How is lubricity measured?

Via a High Frequency Reciprocating Rig (HFRR), of course. This is a steel ball pressed against a flat surface with a fixed load of 200g. The ball is then rapidly vibrated back and forth using a 1 mm stroke. After 75 minutes, the flat spot (wear scar) that has been worn on the steel ball is measured with a microscope. The size of the wear scar (in microns) is directly associated with the lubrication qualities of the fuel being tested. It’s suggested that if the HFRR wear scar diameter is less than 460 microns, the fuel will perform satisfactorily in an engine.

Any old engine oil will work, right?

Nope. The main reason that old-timers suggest 2-stroke oil tipped into the fuel tank is best is because unlike four-stroke engine oil, the 2-stroke stuff won’t turn diesel to gum when mixed in.

What is it about 2-stroke oil that got this story started?

It’s not just to do with the belief that a lower level of sulphur in diesel makes the stuff less lubricating, but also because zinc dialkyldithiophosphate (mineral oils) and zinc di-thiophosphate (ZDTP) for synthetic oils that was invented by Castrol more than 70 years ago is an anti-wear ingredient. But, over the years the concentration levels of the stuff allowed in oil (both 2-stroke and four-stroke) has been reduced because of concerns about health and the environment. See, while the ingredient does afford good metal-to-metal lubricity high concentrations of zinc can foul injectors in modern diesel engines. This is less of an issue in older diesel engines (built before the mid-1980s).

But it’s not just the lubricating nature of zinc, that got this story started…see, in a petrol engine, 2-stroke oil is designed to adhere to metal engine parts once the petrol has evaporated away, keeping everything lubricated. Just be mindful that some pour-in lubricants are intended to be added once a tank and some only every 10,000km.

Oil is a lubricant, so adding it to diesel makes it even slipperier, right?

Wrong. A test conducted in South Africa utilised both the High Frequency Reciprocating Rig (HFFR) and the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE). The former is considered as the industry standard measurement for lubricity. This latter test is intended to determine the point at which boundary lubrication fails (or the fuel’s ability to wall wet of, say, piston rings), while the HFRR test is a better measurement for wear and tear. Together, though, the results tell an interesting story.

First, what is the SLBOCLE method? Starting with a light load applied to the ball full hydrodynamic lubrication will be realised. The process will transition into boundary lubrication conditions as the load continues to rise and the test will finally end when the limits of boundary lubrication are reached, and the ball scuffs the cylinder surface. As the flat spot on the ball grows in size, the point is reached where the applied load is spread over a sufficiently large rubbing area for ‘boundary lubrication’ to take effect. However, because the motion is reciprocating (stop – start – stop), the worn flat spot will continue to grow.

Testing the claim

Now for the test itself. There is so much stuff on the Interweb about tipping 2-stroke oil into diesel that it can be hard to work out what’s true and what’s not. Some claim different types of injectors make it okay to use 2-stroke, others claim it makes engines quieter and saves fuel. But none provide evidence to back up the claims.

There is only one publicly-accessible document about adding 2-stroke oil into diesel and that is from Sasol’s Energy Technology division. The paper was published by one of the company’s engineers, Adrian Velaers, at the 11th International Tribology Conference in South Africa in 2015. It’s not the only paper about lubricants and additives in diesel, but almost all the others are behind very high and expensive pay walls. But the abstracts of all these papers have one thing in common and that is that adding 2-stroke oil into diesel does nothing.

Do go on…

It’s about now, I reckon, that the keyboard warriors will be reading this and going, oi oi, fuel company sponsoring research, hmmm. And, sure, I’m a skeptic too, but the tests were conducted using industry-standard methodology and equipment. The lubricity of several diesel fuel types and 2-stroke oil itself was measured, and, yeah, it’s way more lubricating than diesel; just not when mixed together.

The fuels tested (EN590) align with those sold in Australia (the relevant specifications are DSEWPaC 2009). Oh, and there are plenty of scientific studies into adding lubricants to diesel to improve its lubricity, but they tend to be hidden behind very high pay walls.

Occasionally, some will argue that adding 2-stroke oil to diesel will improve the Cetane number (Octane rating for diesel) but according to the SASOL test it was actually ever-so-slightly less. The conclusion drawn was, “There is no technical reason for trace amounts of a light lubricating oil to materially change the ignition characteristics of diesel. The results follow by indicating negligible differences in cetane number when 2-stroke oil is added to diesel at 200:1”. So, there’s no power benefit from adding 2-stroke, but that’s not the claim by the Interweb.

Not slippery

Rather, lubricity is the claim made by proponents of the 2-stroke oil addition philosophy but in the test conducted by SASOL, the diesel dosed with 2-stroke at 200:1 was a lot closer to the 460-micron maximum in terms of wear than was the regular diesel. Meaning, adding 2-stroke oil to diesel doesn’t improve the lubricity of the diesel.

Looking at the boundary lubrication of 2-stroke mixed into diesel, the results showed the addition of 2-stroke oil ever-so-slightly improved the wall-coating effect. But the researchers called the result negligible, suggesting, while it slightly improved the effect it wasn’t by enough to recommend its usage.

So, what’s the answer here? According to the tests by SASOL the practice of adding 2-stroke into diesel at 200:1 dilution doesn’t tangibly improve the lubrication quality of diesel and in one test, actually decreased the lubricity.

Adding 2-stroke can cause more problems than it’s worth

While, on the surface, it can look like adding 2-stroke oil to diesel might do no real harm or good, that’s not the case. See, because of the fact it contains zinc and diesel does not you run the risk of increasing the risk of injector fouling.

Based on industry standards for injector fouling tests, the researchers at SASOL hooked up a brand-new common rail diesel engine (they don’t specify the size or vehicle it was from) with brand-new injectors. Running at peak power which was 4400rpm for 16 hours the researchers were looking for any drop in power and fuel pressure to indicate injector fouling. Measurements were taken every 30 minutes. This first test had fuel dosed with zinc at 1ppm (parts per million) and contained none of the detergents that regular diesel contains.  Because zinc build-up can break off with fuel flow and thermal contraction (cool down) the engine was stopped after eight hours of running and allowed to cool down. At the end of the test, the engine was showing a drop in power and fuel flow of six percent.

With new injectors, the high-load test was conducted in the same manner for both the regular ultra-low-sulphur diesel and the ULSD dosed with 2-stroke oil at 200:1. The former resulted in an almost zero loss of power or fuel pressure while the test with 2-stroke oil revealed a two per cent drop in power and fuel pressure.

The researchers concluded that even with minimal amounts of deposits forming on the injectors being broken off with the cold re-start, deposits remained, meaning they would continue to build-up overtime leading to “substantial injector fouling over the lifetime of a diesel engine”.

2-stroke is bad for DPFs?

The build-up of zinc on injectors is bad enough, but the deposits that do breakoff during cold starts will eventually make their way to the DPF where they’ll deposit as ash and won’t be burned off. This could result in premature and permanent blockage of the DPF.

There’s detergent in diesel

You’ve all seen the gunk-busting TV commercials petrol companies run, well, no matter what they may claim, all diesel contains detergent which is designed to keep your engine running clean and help to keep deposits from forming on things like injectors and intake valves. Unfortunately, most of the testing on detergents in fuel has been performed on petrol and not diesel.

But a quick whip-around the grounds reveals that all fuel companies in Australia include detergents in all their fuels (petrol and diesel), however, if you’re buying the top-spec fuel (petrol or diesel) from the brands then you can expect a much higher percentage of detergent in your fuel.

And don’t go looking too closely at reports overseas because this can be a minefield. See, in some countries, detergents are added at the fuel delivery stage but in Australia they’re added at the refinery stage.

Bio-Diesel might be the answer?

According to numerous researchers working in Australia, the US, Europe and Asia, tests of adding bio-diesel to ultra-low-sulphur diesel at a level of 1-2 per cent, which is a higher concentration than regular lubricity additives but improves the lubricity of the diesel by 30 per cent. A report by the CSIRO suggested adding one per cent of bio-diesel could improve lubricity by up to 65 per cent, but we couldn’t find any other literature to support that claim – although a lot of research into this subject is, unfortunately, hidden away behind pay walls.

But running neat bio-diesel in a diesel engine has its own problems in that its cloud point (the point at which it becomes wax and unusable) is much higher than regular diesel. The other issue is that while bio-diesel tends to have the same Cetane number as regular diesel, it produces less heat which could cause problems for DPFs.

Testing in the US, at the Worcester Polytechnic Institute has revealed the use of bio-diesel can see a reduction in tail-pipe emissions compared with regular diesel. While the CSIRO and others have suggested that while particulate matter can be bigger with bio-diesel it is still captured by a DPF and burned but with less volume of particulate matter when compared to regular diesel.

Only bio-diesel isn’t the answer…

While there are many benefits to running small amounts of bio-diesel in regular diesel, like lubricity and reduced emissions, many vehicle makers will either not allow the use of bio-diesel in their vehicles or only allow B5 which is regular diesel containing five per cent bio-diesel. That said, the Australia Government is looking into the introduction of B20 or, at least working on an acceptable framework, and reckons that bi-diesel could power around six per cent of all diesel vehicles here by 2030.

My personal vehicle is a diesel and it’s not compatible with bio-diesel in any way shape or form. But our Ford Everest long-termer’s owner’s manual says: “Your vehicle is suitable for bio-diesel blends of up to 7% (B7). You can achieve acceptable engine performance and durability using B7 by adhering to the below guidelines. Do not fill the fuel tank with fuel that contains biodiesel with blends greater than 7%.

Note: Do not use raw oils, fats or waste cooking greases, these are not bio-diesel.

Note: If your vehicle experiences low temperature waxing issues, consider using different diesel brands or diesel with lower bio-diesel content.

The manual also goes on to say that bio-diesel can degrade quicker than regular diesel when stored for long periods of time or when exposed to warm and/or humid weather. The other gem in the owner’s manual is this one, “The use of aftermarket fuel additives or other engine treatments are not recommended”. Hmmm.

This is not an additive bash.

This article never set out to explore the use of fuel additives, boosters or cleaners (because we all know corrosion inhibitors and bug and algae killers in diesel work), rather it was aimed at the idea that adding 2-stroke oil to your diesel will improve the lubricity. And it turns out it doesn’t. At all.

We took this question to our Facebook page and plenty of people told us they’d mixed the stuff with diesel to improve the lubricity because the reduction of sulphur reduces the lubricity of diesel. So, go ahead and keep using it if you feel you need to, but just know that adding 2-stroke oil to your fuel tank is doing absolutely nothing for your vehicle and will likely end up increasing injector fouling.

Some will say that because of how hot a diesel engine burns running, say, JASO-FC 2-stroke might not help with lubricity, but it will help remove carbon deposits. And, as we all know, carbon deposits (as a by-product of the fuel burning process) can sap power and see a diesel engine use more fuel because it’s all gunked up and struggling. Like you trying to run a race with a blocked nose.

But, all fuels, petrol and diesel, in Australia have detergents added that are ‘designed’ to keep carbon deposits to a minimum, but stop-start cycling, short trips and even excessive idling can cause a huge build-up of carbon and other deposits on intake valves. Tipping 2-stroke oil into your fuel tank won’t help here. Using a high-quality engine oil can help, perhaps some additional cleaner (depending on whether you’re buying top-spec diesel or not) and so can fitting a catch can which can help capture any waste oil before it settles and gives deposits something to stick on to.

So, do I add 2-stroke or not?

As the SASOL test concluded, adding 2-stroke oil to ULSD does nothing to improve its lubricity; there are already additives included that help with that. And that it might even cause increased injector fouling over time (increased by about two per cent). So, you could argue that it doesn’t really help or hinder…but, for me, I wouldn’t bother wasting the extra money on a bottle of 2-stroke to tip into my diesel. You’re probably better off adding some sort of high-strength cleaner instead – there’s evidence around to suggest that stuff does work. But that’s another story for another time, because I need a lie down… and you probably do to. Thanks for making it to the end of this article.


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