somethin I was told is the first number is how thick it is at start up - the thicker it is the longer it takes to travel through motor and the second number is how thick it is when heated- the thicker it is the more protection at high revs . my interpretation of this,is that a small first number and a big last number is better eg. 5w50 compared to 15w40. I could be wrong though,I'm sure if I am someone will shed some light on the matter