I believe that GL1 90 weight gear oil is only recommended for winter (below 32 degrees.) Above 32 degrees the recommendation is for 140 weight in both the transmission and differential. NAPA only has the GL1 straight mineral oil in 90 weight. I found a source for a 140 weight mineral oil that sounds promising. It is sold by a supplier that provides oils for steam railroad operators. Their web site is steamenginelube.com and the oil name is PBJ460. I spoke to a representative from them last week and he told me that this product is the same weight as an SAE 140. I haven't checked out the web site yet so I don't know any more about it. I hope that this info helps as there is a lot of confusion about the proper oils to use in our cars and trucks.
Roger List
Posted on Nov 1, 2009, 9:46 PM from IP address 67.177.192.92