Here is some more information that comes to the same conclusion.

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.

The above can be found at:
http://www.penten.com/faq/faq.htm#What%20happens%20if%20API%20GL-5%20gea
r%20oil%20is%20used%20in%20an%20API%20GL-4%20gear%20oil%20application...

Richard

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Zoran Mladen
Sent: Saturday, March 22, 2003 10:56 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Mobil 1 Gear Oil - Can I use this?

So I picked up some Mobil 1 Gear Oil tonight with
plans of putting it in an 87 Syncro.  The bottle
simply says exceeds GL-5 standards.  Being that we
need GL-4 in our trannies, can I use this?

Z

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