Compare types of oil using the product's composition, mileage rating and additives. All quality motor oils carry a service standard certification assigned by the American Petroleum Institute in a small starburst logo on the container. Gasoline engine oil uses the "SL" API standard, while diesel oils use a "C" standard.
The first factor to consider when comparing oil is the product's composition. Motor oils are either conventional, synthetic or some mix of the two. Conventional oils are the most affordable, mixes are slightly more expensive and full synthetic oils have the highest prices.
Conventional oil is suitable for most new and recent cars and trucks; it is designed for normal engine speeds and moderate temperature ratings. Synthetic oils are designed for high-performance engines, and they have superior performance to conventional oils in virtually every way. Mixed oils add a small amount of synthetic oil to conventional oil, slightly improving its performance while keeping prices relatively low. Mixed oils are best for use in off-road vehicles and vehicles in demanding service applications.
Oil additives have a number of functions that can improve an oil's performance or its ability to function in adverse situations. High-mileage oils have added seal conditioners that prevent old, brittle oil seals from cracking or breaking. Dispersants and detergents keep wax and dirt particles suspended in engine oil from forming deposits. Pour-point depressers prevent oil from thickening in extremely cold temperatures.