<VV> y'all

[Padgett]

>  The multigrades didn't have different weights.  As someone else said, 
> the oil manufacturer added long chain polymers so that as the oil reached 
> operating temperature it wasn't any thinner that the target 
> viscosity.  So a 10W-30 wasn't any thinner than a 30 weight at operating 
> temperatures.  The problem is that shearing and other wear factors "cut 
> up" the long chain polymers and you didn't keep the higher viscosity as 
> time went on.

Exactly . 10W30 means at at -18C the oil is within the viscosity range for 
a 10 weight oil and at 100C it is in the range of a 30 weight oil. Over 
time it approaches the characteristics of a straight 20 weight oil. This is 
what I meant when I said that on some cars I could tell when it was time to 
change the oil by a 2-4 psi drop in cruise oil pressure (all of my cars 
have oil pressure gauges, I was infested by Jaguars in my yout.).

Also for the last 100 miles or so before an oil change I would sometimes 
toss in a can of ATF. About 20 weight and quite a remarkable cleaner.

Racing oils generally do not have viscosity improvers so much as additives 
to resist heat and remain stable.  Do not know if it still exists but when 
growing up I used to love the smell of Castrol R.

Padgett