Viscosity Index numbers?

[2003f7]

What do the Viscosity Index numbers mean? I have been comparing M1 5W-30 to RP 5W-30 and I see that RP is 157 and M1 is 169. What does this mean?

 

[deven]

Here is a good article for understanding VI and VII.
http://www.tpub.com/content/engine/14105/css/14105_37.htm

There is a reason why RP UOA's show the oil thinning out prematurely without any inherent problems with the engine.

 

[ProfPS]

There are too many variables that attribute to the VI spec: adpack chemistry, base oils, % of each to name a few. Don't place too much on this spec. There are dino and syn oils that have low VI's, turned good UOA's, and users have had engines lasted a couple hundred thousand miles without a problem using a low VI oil.