okay this might be techie but i'll say it anyway and most of you probably already know this.
the oil grades commonly used in this country are based on the SAE (society of automotive engineers) index. and are typically xxWxx (eg. 10w40, 0w40)
the first number is the viscosity of the oil when it is cold (the "w"means winter). and the lower this number is the better. most engine damage is done when the engine is cold because oil is vicous and is more like treacle than oil.
the second number is the viscosity of the oil when it is hot and in most respects the oil is pretty much always gonna be 30, 40 or 50. (most petrol cars require 40, and diesel pd's need 30)
it is the winter value i would take note of, the lower this number is the more runny the oil is and the faster the oil will lubricate your bearings and other things. the oil will also heat up faster so the bearings will cool down and you get sufficient hydrostatic pressure on the white metal bearings.
also 15w40 is usually mineral oil (refined crude oil) although it can be sometimes semi synthetic (i.e partly made by people in white coats). 10w40 is pretty much always semi synthetic. 5w and lower is pretty much always fully synthetic.
My personal preference is mobil 1 0w40 (when i had a petrol car) it is expensive but on cold days there is a definite improvement on the speed at which the engine heats up and the noise is reduced to. also fuel consumption on short journeys is improved. just my opinion but i thought i would share it anyway
hope this helps
sanj
