Here's a pretty descent info page on what the viscosity numbers mean.
http://www.upmpg.com/tech_articles/motoroil_viscosity/Go to the page to see the full chart, I've reduced it so it's readable here.
SAE Viscosity Chart (High Temp)
100° C (210° F)
20 5.6
30 9.3
40 12.5
50 16.3
60 21.9
Winter or "W" Grades SAE Viscosity
Low Temp (-15 to -40°C) Viscosity cP
0W 3.8
5W 3.8
10W 4.1
15W 5.6
20W 5.6
25W 9.3
Quote:When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 210° F or 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C). The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.
I hope y'all never see -15
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