Cans of gasoline engine oil usually have an American Petroleum Institute (API) starburst label that certifies their use for gasoline engines. This distinction may seem odd but not all motor oil is good for gas engines. The difference is especially of interest to those who notice that diesel oil is much cheaper and would like to use it in their gas engines to save money.
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The catalytic converter is a porous metal filter located along the exhaust line between the engine and the muffler. The metal matrix inside speeds the conversion of toxic emissions into benign products. Some emissions however, can damage the converter's ability to perform this catalysis, such as lead, zinc and phosphorous. A failing catalytic converter can lead to your car's failing an emissions test. Since diesel engines don't use the same kind of catalytic converter, diesel oil has a higher zinc dialkyl dithiophosphate (ZDDP) level than gasoline engine oil, to form a film on bearings and the camshaft to prevent wear. ZDDP contains both zinc and phosphorous. This content is the leading reason diesel oil can't be used in gas engines.
Diesel oil is more viscous, in other words heavier or thicker. This is needed because there's more space between rod and crankshaft bearings in a diesel engine. Thin oil would drain from these spaces. Obversely, putting too thick of an oil in a gas engine will make it run hotter. Heavier viscosity is bad for a gas engine on start-up and in cold weather. You would also loose compression of the oil, slowing its circulation into the engine.
The level of detergents and other additives is higher for diesel, to combat the higher soot levels of diesel engines and take advantage of the difference in the catalytic converter in diesels. In a gas engine, diesel oil's greater concentration of detergent, or scrubbing agent, can remove manufacturer's deposits on the cylinder wall that help seal the combustion gases. This can lead to smoking inside the engine, which can in turn damage the catalytic converter.
Some motor oil additives serve to lower the viscosity on start-up and raise it at high temperatures. Diesel oil has a longer lifespan because of the higher level of such additives. Specifically, higher additive levels in diesel oil means you can drive further before they degrade to levels too low for a gas engine. Proper viscosity at proper temperatures prevents wear and strain on the engine. The extra additives in diesel oil don't help much in a gas engine, since diesel oil is usually sold at higher viscosities than gas engines use. The viscosity is therefore mismatched right from the start.
Use of Diesel Oil in Gas Engine
You can use diesel in cars older than model year 1975, when catalytic converter were first mandated. You must match the viscosity, which could be difficult considering the higher viscosity requirement of diesel engines. Too high of viscosity, in a gas engine, would provide poor protection on start-up and during the winter. The higher viscosity would also lower gas engine fuel efficiency slightly. If you do decide to use one type of oil in the other type of engine, you can tell which is for gas engines by the "S" in the oil can coding ("S" for "servicing"). "C" is for diesel engines ("C" for "commercial").
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