Here's a tidbit on the subject:

GL4 vs GL5 from pennzoil's site:

What happens if API GL-5 gear oil is used in an API GL-4 gear oil
application?

API GL-4 and API GL-5 products typically use the same extreme pressure
(EP) additive system, with the API GL-5 having about twice the
concentration of a API GL-4. In service, these additives become active
under extreme load and temperature when the protective oil film can be
squeezed away. EP additives work by forming wear-resistant compounds with
the metal of the gear tooth surface. As the gears mesh, these compounds
shield the gear teeth from direct metal-to-metal contact that would cause
wear and damage to the gears. If too little of the active additive is
present, proper protection would be compromised. Too much of this additive
could cause excessive chemical corrosion of the gear surface. If an API
GL-5 gear oil is used in a application where API GL-4 gear oil is called
for, chemical corrosion of "yellow metal" components may occur, such as
bronze synchronizers, brass bushings, etc. This may lead to shifting
difficulties or shortened equipment life.

-Dana-

>
> Yeah, agreed, we've talked about this alot.  What I read most recently was
> that GL-5 had some component (sulfur, I think?) that ate synchros about 50%
> faster than GL-4.  Is this true, and is it true of GL-5 synthetic as well?
> Hmmm....
> bmc  :)
> "Faith will move mountains, but you'd better bring a shovel...."
>
>  This thread keeps coming up. GL-5 is a higher performance standard than
> > GL-4. The concern is not that GL-5 is more corrosive, but may keep the
> > syncros from working properly due to the higher extreme pressure
> > capabilities. Since bearing failure and Ring/Pinion failure are both
> > common and expensive, I myself use the GL-5 synthetic oils. Syncros are
> > cheap compared to those items.
> >
> > Dennis
> >

> > quarts for the syncro.  Both call for GL-4, not GL-5.  In the syncro
> > tranny,
> > GL-5 eats the synchronizers, does anyone know if that's true on the 2WD
> > as
> > well?
> > tx,
> > bmc  :)