13 Sep 2003, 20:01, Roy J. Tellason (1:270/615), wrote to Tom Walker:

Hi Roy.

 TW>> I just bought a 1998 Chev S10 Pickup and note that GM strongly 
 TW>> recommends against using any oil heaver then 30 Weight. 
 TW>> Interesting, Cannot immagine why.

 RJT> Tighter tolerances?  Beats me...  I know that with most of the 
 RJT> stuff I have that has lots more miles on it I go with the 
 RJT> 20W-50,  too.  The one car hasn't gone that far up there yet,  
 RJT> so we still put 10W-40 in that one,  for now.

EPA is the keyword!  If you read the charts (not the text) in the owner's
manual you'll find something like 'SAE 5W10 is recommended for the Arctic',
5W20 for Canada and northern USA, 10W30 for mid-USA, 10W40 for southern
USA, and 20W50 for Mexico and the Carribean states'.

This info directly contradicts the "EPA stickers" found all over
under the hood.  

Factory tests for EPA requirements are conducted with 5W20 at the factory
(under very controlled conditions), and they barely squeak by.  Using an
oil heaver than SAE 10W30, puts a _slight_ drag on the engine when cold,
and the computers cause the engine to run rich, thus flunking the EPA
requirements.  To hell with what's best for the engine in it's own real
world climate!


     Good luck...  M.

--- Msged/386 TE 06 (pre)