The idea that GL-5, and to a lesser degree GL-4 oils are corrosive does not seem correct. That would mean they were effecting the yellow metals even when sitting in storage. The best information I could find about the effects of the additives in GL-4/5 oils is this ;
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is bound to.
I have no expertise in this area but it would seem to me that the only time these oils can have a negative effect is during shifting. Perhaps that is why Novak, a generally acknowledged expert in SM420 transmissions says GL-5 is fine in these transmissions.
I will probably never put enough miles on my truck for this to become an issue but to use a phrase I have come to dislike, using "an abundance of caution" I am probably going to drain my GL-5 and replace it with something not known to cause problems.
