I assume a good quality of gear oil, but I read somewhere that you do not want to use GL-5 with brass synchros. Would a good semi synthetic 85-140w be appropriate?

I don't see why not.

Use only GL-4 or you'll be sorry.

Where do I find GL4? It isnt at Checker...I looked at everything and it all said GL5 compatible...what about royal purple synchromesh?

Only place I've found a GL-4 is O'Reilly, and it's a cheap store brand. They usually look at you funny and try to sell you the GL-5 anyway, but if you press the issue they'll find a gallon in the back covered with dust.

It's a little hard for me to believe that everyone out there running GL-5 in their transmissions is eating their synchros and that the oil makers haven't corrected this problem (if it was more than an urban legend in the first place), but I figure the GL-4 is actually a little cheaper than the newer stuff, my old transmission was made to run with it, and I change my fluids frequently enough to not worry about the added benefits a GL-5 or a synthetic might have.

I'm a GL-4 man!

Might not be an urban myth.

http://rds.yahoo.com/_ylt=A0geu.3wZTxH4RsBj0hXNyoA;_ylu=X3oDMTE4ZmVxaTZ nBHNlYwNzcgRwb3MDMQRjb2xvA2FjMgR2dGlkA0Y2NjZfOTEEb ANXUzE-/SIG=13o3hbnu5/EXP=1195226992/**http%3a//www.pzlqs.com/Tech/Pdsheet/DomesticMarketing/Gear%26Transmission/pdf/GearplusSAE80W90GL-4.PDF

A Google search on "yellow metal GL5" also turns up a huge amount of information. As with everything else on the Web some of it is conflicting information, but the general consensus seems to be that GL-5 gear oil and yellow metal are a bad combination.

I did learn that some GL-5 gear oils contain buffering additives that reduce the effect on yellow metal, but unless you can confirm that your brand of choice is buffered, it's probably best not to use GL-5.

From Redline Oil:

Most manufacturers of manual transmissions and transaxles recommend an 80W
or 90W GL - 4 lubricant. GL - 5 gears oils which are required in hypoid
differentials are not used in most synchromesh transmissions because the
chemicals used to provide the extreme pressure protection can be corrosive
to synchronizers, which are commonly made of brass or bronze. Typically,
the use of a GL - 5 lubricant in a synchromesh transmission will shorten
the synchronizer life by one half. The extreme pressure requirements of
spur gears and helical gears found in transmissions are not nearly as
great as found in rear-wheel drive differentials. A GL - 4 lubricant
provides adequate protection for most manual transmissions, unless a
unique design consideration requires the extra protection of a GL - 5.

From Penzoil:

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.

From Castrol:

API GL-5 gear oil designates service of hypoid and other types of gears
commonly found in rear axles. A GL-4 oil may be employed in certain manual
transmissions. These differences are mainly distinguished by the level of
active additives employed in the product. Different applications require
different additive levels and different component materials can be
affected by the levels of additives present in the lubricant. Having said
that, GL-4 and GL-5 gear oils are not compatible and manufacturer's
recommendations should be adhered to. They cannot meet the same
specifications and the product performance and tests are different.
Typically, a GL-5 gear oil will have about 2 times the active additive
level of a GL-4 product. This additional additive can cause problems with
yellow metals like brass or bronze.

From Penrite:

Some modern additive packages used to produce GL5 rated hypoid gear oils
can be corrosive to yellow metals and so in cases where the incorrect oil
has been used, it is advisable to change as soon as possible.
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