What's the deal with oil ratings?
 

I understand that the difference between 10W-30 and 10W-40 oils have something to do with viscosity, but what exactly do each of the numbers and letters mean? And which is ideal for which application?

John

Multiviscosity oils are identified by their viscosity spread. 10W-30 is essentially 10-weight oil that becomes no thinner than 30-weight oil when heated to operating temperature. 0W-20 is 0-weight that doesn't get any thinner than 20-weight when heated. Lower numbers are better suited to lower cold-start temps. I doubt you would ever need 0-weight oil in Maryland. Up nahth (;)), however, it's probably wise to run 5W-something in the winter.


With organic oils, generally the wider the spread, the less stable it is.