It is a by product of refining process and has varying grades and that's before they add anything.
Multigrades are just that, they are designed to flow at low temperature and molecules expand with heat giving enhanced properties to keep metal surfaces apart and lubricated as they expand.
I use a semi- synthetic because as falco9 rightly says it's false economy to use expensive oils unless you want extended service times or race.
So 10-40 or 15-50 grade ? personally I use 10-40 in winter and 15-50 in summer, that way I feel I have it covered.
There is a valued argument in the light of recent hot summers that 05-50 full synthetic is a fits all motor oil.
I work on the basis that if you leave oil changes longer then the oil will hold more dirt and metal particles in suspension, dirt is not good so I change my oil more often
Full synthetic oil if there is such a thing tends to be enriched with all kinds of friction enhances to aid fuel consumption amongst other things, this is why it tends not to be good for wet clutches it builds up on the plate surfaces causing slippage, most are designed for cars that don't use them.
I personally think Mobil 1 motor oil is one of the very best but I wouldn't use it in my new engine because it is so good you will probably never get it to bed down properly.
OK incoming
