Oil is oil, right? If you have a diesel engine and you need an oil change, what’s stopping you from throwing in the same kind of oil you put in your gas-powered vehicle?
In the broadest sense, gas and diesel engine oils have the same anatomy or makeup. They are formulated from the blending of 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 is fitted with a catalytic convertor that converts its toxic emissions into more environmentally-friendly components. Engine oil has long contained ZDDP as an extreme-pressure additive. ZDDP (a combination of zinc and phosphorus) is highly effective and very good at what it’s supposed to do. But if and when some of that oil gets burned, the ZDDP breaks down into components that basically destroy the catalytic convertor’s ability to do its job. For this reason, the EPA has successfully exerted pressure on the lube industry to reduce the amount of ZDDP in engine oil, but they can’t eliminate it completely (because there's no good alternative).
So one reason why you don’t want to put gasoline lube oil in your diesel engine is because engine oils for gas vehicles don’t have as much ZDDP in them. Diesel lube oil have higher amount of ZDDP because their catalytic convertors are designed differently, to be able to deal with this problem. So diesel engines rely on sufficient amounts of ZDDP present in the lube oil as an extreme pressure additive. Gasoline engine oil don’t have the same level of “EP” protection. And, of course, there's problems with the converse as well. Putting diesel oil in a gasoline engine puts too much ZDDP into the system, which will shorten the life of the catalytic convertor in that vehicle.
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. You would not want to put a diesel oil in a gasoline engine because it would not flow well enough in 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. And 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 a 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 isn’t able to clean up. Not a good situation.
What about the opposite – what if you put a 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 kind of oil in your diesel engine. Check the API doughnut on the label. Diesel oils with be labelled with a “C” for compression-ignition, whereas gas engine oils will be labelled with an “S” for spark-ignition.