Posted: Apr 21 2008 at 6:35am | IP Logged

A note from the old chemist.  Syntetic oil is of two fundamental types.  One, typified by Castrol, is produced from hydrotreated petroleum.  That is regular dino oil that has been hit with very high pressure chemistry to split the molecules and react out the double bonds.  Double bonds are what causes old oil paint to "dry" or cure and the presence of these is what causes varnish and sludge in engines.

The other type of synthetic uses truly synthetic molecules such as PAO (polyalphaolefins) and/or polyesters, such as are found in jet engine lubes oil.  Mobil 1 and Amsoil are of this type.  These are engineered with particular moecular sizes, no double bonds and require MUCH lower levels of viscosity improvers that are common in regular multi-weight oils.  These will flow better at low temps, are chemically very stable and have very high lubricity.  This allows for slightly bettert gas mileage.

Now, all of that said, modern petroleum-based lubricants are also very good.  If you change your oil at a decent interval, they are fine.  I use synthetic as I change my own and get 5 qts of Mobil 1 at Wal Mart for $22.  I leave it in my Frontier for about 4000 miles (under warranty).  In my wife's 5-year old Saturn, I just do 2 changes/year. 

Remember, even synthetic oils will pick up dirt that gets through the air filter and blow-by chemicals that are produced by the combustion of gasoline.  I just don't like leaving an oil in there for 25,000 miles for the sake of a couple of oil changes.  Just me.  Also, make sure that the oil you use does not have high levels of zinc anti-wear additive as this can really degrade a catalytic converter.  Mobil 1 is safe in this regard, for example, while some others are not.  AT the price of converters, saving on an oil change only to have to replace your converters can be a bad idea.