Originally Posted By: suleman
I'm new to this oil thing. Can you can explain me difference between 5w and 0w. What do you mean by cold start.
The number in front of the W is the cold temp performance of the lubricant measured at extremely cold temperatures. The two tests employed are CCS and MRV; Cold Cranking Simulator and Mini Rotary Viscometer.
As an oil cools it get heavier, once around the -10 to -15C range, the viscosity will basically double for every 5C drop in temperature measured using either CCS or MRV (the numbers are not interchangeable between the two methods).
There are limits imposed for each Winter (W) classification for both CCS and MRV for a given temperature as per SAE J300:
Basically, a 0w-xx thickens less as the temperature drops than a 5w-xx, which thickens less than a 10w-xx....etc. However, as long as you are within the range that is acceptable for a given grade (as per the chart above) the oil will pump and performance between that lubricant and one with a better W rating is not overly important unless you have a weak battery or an engine that has an issue moving extremely heavy oil due to a design issue.
If you want to picture it, think of a graph with the CCS viscosity along the one axis and temperature from 0C down to -35 on the other. At the beginning of the graph the viscosity of the two plots will be very similar but as you get down further the curve for the oil with the poorer W rating will begin to get sharper as it meets the CCS limit for its grade at a higher temperature. The lubricant with the better W rating will have a more progressive curve as it will be under 6,200cP @ -35C.