Did you know that while the engine oils for gasoline and diesel engines may seem identical at a glance, their unique demands paint a different story?"
Engine oils for gasoline and diesel engines are the same, yet different simultaneously. At their core, they both have the same basic makeup in their base oil. However, the differences come when we consider the desired performance needed for diesel engine oil, which may differ from that of a gas engine.
In the broadest sense, gas and diesel engine oils have the same anatomy or makeup. They are formulated by blending base oils and additives to achieve a set of desired performance characteristics for their particular kind of engine. From this simple definition, we start to diverge when examining the lubricant's required performance for each engine type.
Every vehicle comes equipped with a catalytic converter, a crucial component for transforming harmful emissions into less harmful ones. Engine oils often include ZDDP (zinc and phosphorus) for their exceptional extreme-pressure (EP) properties. However, ZDDP can degrade into substances that impair the converter's functionality when oil burns, posing a significant problem. Consequently, the EPA has pressed the lubricant industry to lower ZDDP levels, though complete elimination isn't feasible due to the lack of alternatives.
This dilemma partly explains why gasoline engine oil, which contains less ZDDP, shouldn't be used in diesel engines. Diesel oils have higher ZDDP concentrations to accommodate their converters' design, which can handle these components, thus relying on ZDDP for adequate EP protection. Conversely, using diesel oil in a gasoline engine introduces excessive ZDDP, potentially reducing the catalytic converter's lifespan in gasoline vehicles.
Diesel engine oils also have higher viscosity ratings than gas engine oils. This matters when considering the pumpability of the oil – how well it flows at the lowest start-up temperatures.
One reason you would not want to put diesel oil in a gasoline engine is that it would not flow well enough for cold weather starts to provide the right kind of protection before the engine warms up.
All lube oils, whether for gasoline or diesel engines, contain essential additives that help them do what they need to do. This is perhaps the biggest difference between the modern oils of today and the engine oils from decades past.
Diesel engine oil has more additives in it than gasoline engine oils do.
Case in point – diesel engines tend to produce more soot and combustion byproducts than gasoline engines do. So the oil in a diesel engine needs to be able to handle this increased load. Higher levels of detergent additives, especially “overbase” detergent additives, in the diesel oil help the diesel engine handle this successfully. So what would happen if you put gasoline lube oil in a diesel engine? There wouldn’t be enough detergent present in the oil, and you’d get wear and other adverse effects from the excess soot that the oil cannot clean up. Not a good situation.
What about the opposite – what if you put diesel oil in a gas engine? Again, not a good situation. The excess detergent in the diesel oil would try and clean the cylinder walls too much, destroying the ring seal and giving you lost compression and low efficiency.
So be sure to use the right oil in your diesel engine. Check the API doughnut on the label. Diesel oils labeled “C” are for compression ignition, whereas gas engine oils will be labeled with an “S” for spark ignition.