Opinions on synthetic manual tranny/diff fluid vs gear oil?

[free5ty1e]

I've done some searching, and really couldn't find anything specific about manual transmission fluid. I've been advised to fill my new JDM tranny with Redline MTL fully synthetic fluid instead of the 80W90 gear oil I was using on my OE 'mission. This stuff is expensive, and it sounds nice, and I know I need 3.5 quarts of it... Do we have an LCO consensus on the benefits of synthetic fluid vs gear oil for our transmissions? What do you guys think, any experiences that might be helpful?

Would I be wasting my money on this stuff or would it be a good idea...? (and of course, why)

[free5ty1e]

Also... should I put the same stuff in my new rear 4.11 LSD? The front differential shares fluid with the 5MT correct? So how much of this stuff do I actually need?

[free5ty1e]

Wait... I'm reading up on these GL ratings, and Redline MTL is a GL-4 oil. Do our transmissions require GL-5 oil? I'm confused.

My front and rear differentials are specified as "hypoid" in the 1992 FSM, so does that mean they need GL-5? And since the front differential shares fluid with the tranny, that means the tranny and differentials all need GL-5, and the same stuff?

From http://www.racerpartswholesale.com/redtech4.htm :

"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."

[Adam West]

Uh,...Scotty's cocktail...check out my writeup and recipy in the reviews section...

[free5ty1e]

.....Thank you, Mayor West. I was not aware of Uncle Scotty nor his fantastic transmission cocktail. You've saved my sanity.

For now.

[silver03]

Back from the dead thread. I just put Valvoline non syn 75w-90 in the rear diff for a scheduled 30k interval change. Our car is a 94 L awd 5 spd (135k). What can I safely and economically put in the front for tranny/diff fluid?

[James614]

Generally any GL-5 gear oil of the proper weight will be fine. Some work better with the synchronizers than others though. Subaru's Certified Gear Oil is a good choice if you want to spend lots of money. From my readings it seems Subaru stuff and Amsoil Severe Gear seem to be the favorites. Valvoline and Mobil make decent regular priced gear oil.

I wouldn't recommend going with Uncle Scotty's Cocktail in a regular DD as it causes accelerated synchro wear and needs frequent changing.

[silver03]

proper weight? 80-90? 75-90?

[James614]

IIRC synthetics its 75-90, mineral oil is 80-90.

[mike-tracy]

Either range should work fine.

[Legacy777]

75-90 is the factory recommend weight, but 80-90 will work as Mike pointed out.

[AWD_addict]

I've found Subaru Extra-S to be better than Redline 75w90 (NS or regular), when cold or hot. Redline didn't seem to work as nicely with the synchros. Regular Valvoline was way worse than that. This is in a transmission from a BD with lots of miles on it.