What Does 2-Stroke Oil Do To A Diesel Engine?

The modern diesel engine is far removed from its more "industrial" forerunners. The current generation is cleaner-burning, more fuel efficient, quieter, and more durable. While there are several reasons for this durability, the fact that diesel fuel also acts as a lubricant is an important factor. However, if we take a step back and look at the cleaner-burning aspect of modern diesels, we hit upon a bit of a conundrum: one way emissions were reduced was by switching to ultra-low-sulphur diesel, and sulphur played a central role in making diesel fuel naturally "slippery." 

In short, the shift to cleaner fuel was more than just an exercise in taming what emerged from the exhaust; it also had an effect inside the engine. Diesel still requires a certain amount of lubricity to protect components like pumps and injectors, and older fuels supplied that naturally. When sulphur was removed, some drivers took to adding two-stroke oil to their tanks under the belief that it restored this lost lubrication. This might seem like good logic; after all, two-stroke oil is designed to burn cleanly and provide lubrication, so on the face of it, it's a win-win situation. 

However, in reality, the facts provide less compelling reasons for adding a little DIY lubrication to your fuel. According to a paper published by Sasol Energy Technology, adding two-stroke oil to diesel does not improve lubricity. Instead, it can introduce zinc deposits that can foul modern injectors. Let's have a closer look at why adding two-stroke oil to diesel might not be the best idea. 

Modern diesel engines are built to high tolerances and require the fuel to be engineered to meet these demands. This is where the idea of adding two-stroke oil to your diesel begins to sound less like a good idea. While the main assumption is that two-stroke oil will add back some of the lost lubrication following the switch to ULSD, controlled testing suggests this isn't the case. At least, certainly not at the 200:1 often quoted as the correct "recipe" for adding two-stroke oil to diesel. (For comparison, two-stroke engines like Ryobi's require a fuel-to-oil ratio of 50:1.)

According to Sasol's research, two-stroke oil added to diesel at the 200:1 ratio made no difference to the lubricity of the fuel in industry-standard testing. Also, the same research found that there was no difference in the cetane number, an indicator used to measure ignition quality. In other words, the fuel doesn't burn better, lubricate better, or protect the engine more effectively when you add a little two-stroke to the mix. 

Adding two-stroke oil to your diesel engine can also bring some downsides. Adding extra zinc to the mix is one of these; zinc and modern diesel injectors are not a match made in heaven — and it doesn't take much. Zinc levels as low as one-part-per-million can foul injectors, and two-stroke oil often contains zinc levels as high as 16 ppm. 

Putting two-stroke oil in your engine won't bring it to an immediate and grinding stop, nor will it bring any measurable benefits. In the longer term, it may even be detrimental. So, if improving lubricity is the ultimate goal, one proven approach is biodiesel blending. Adding biodiesel at a level of 2% can provide improved lubricity. However, before you rush out and get a supply of biodiesel, it's worth noting that most U.S. diesel is already blended with at least 1% biodiesel, and while you can mix in more, there are some drawbacks to be aware of before you blend biodiesel and diesel. Among the downsides of biodiesel are a lower energy density and cold weather gelling, and it can also degrade components like rubber hoses and gaskets. 

It's also worth noting that fuel manufacturers and engine designers weren't oblivious to the problems of ULSD and didn't just dump it on the market with their fingers crossed. Other additives were developed, and along with the aforementioned biodiesel, these helped to replace the lubricity that was lost when sulfur levels were restricted. Diesel fuel additives work and are there for a reason. On whether two-stroke oil can be added to this list, the best we can say is that the benefits are debatable and the downsides — while probably not catastrophic in the short term — do definitely exist.