Believe it or not I still get a lot of questions about motor oil from housewives to car nuts. And one of the most common question I hear is "What is the difference in motor oils?" Well for this discussion I am going to focus on the highlights of the two types of oil that most of us use everyday. Pure Base Oil (API SA) and Conventional motor oils (API SM).
First is the conventional motor oils, lets keep to the basics for this article. Most conventional oils are petroleum base. This means that they are made from petroleum.
Sorry to be redundant there but that is a fact. The manufactures of these oils add different additives to the oil so that it can meet the daily grind of being used everyday. So you ask what these additives do. First and for most any oils job is to reduce friction. Which in turn reduces temperature, prevents rust, and helps to keep the engine clean from combustion deposits. Now for a little technical jargon.
Conventional oils that are Pure (meaning no additives at all) do not provide protection against most contaminants, these oils in question would be API SA oils (these oils do not have any additives), which are not suitable for modern engine applications. However modern Petroleum oils like the latest API SM category do contain detergents and other chemical additives to "keep contaminants in suspension". The theory is that when the Petroleum oil is changed frequently these contaminants will drain out at each oil change. This is the main reason why extended oil drain service is not recommended with any Conventional oil lubricant, so that being said change any petroleum base oil between 3 thousand and 5 thousand miles.
Source by Tony Manning