Originally Posted By: TurboFiat124
The GL-4 subject caught my attention. I just recently bought a Citroen 2CV and it says to use GL-4 EP in the transmission. The only place in the US I could find it was Amazon.com . I mean I could order it from Europe but the shipping is expensive and wanted to do it now before building a wish list.
https://www.amazon.com/Sta-Lube-SL24239-GL-4-Multi-Purpose-Hypoid/dp/B000M8RYMC
GL-5 is supposed to contain sulfur which will attack the brass synchronizers. I've owned Fiats for about 25 years and the transmissions calls for GL-1 90. Sort of hard to find at autopart stores but widely available at tractor stores like Tractor Supply and is actually cheaper than GL-5.
Just like the HLP-68 my Trabant uses which ended up being AW-68.
Just about every Fiat I've bought has had worn synchronizers. Except for my Yugo. Once I had the transmission overhauled, and put the correct oil in it, they did just fine.
I suspect allot of owners put GL-5 in the transmissions out of ignorance which led to the synchronizers early death.
When I drained the gear oil from my Citroen it had the same pungent smell of GL-5. I tend to hear gear a bit of clash when going from 4th to 3rd and sometimes 3rd to 2nd. The car only has 24,000 miles on it so I wouldn't think the synchronizers would be worn this early.
I got to where I just blip the throttle with the clutch pressed in before downshifting which seems to do the trick.
Funny that the Citroen has no gear clash when going up through the gears, just downshifting.
The Fiats wanted to grind when going from 1st to 2nd. Double clutching would prevent this but no amount of double clutching works on the Citroen. Just blipping the throttle when the clutch in. I guess this aligns the gears somehow.
This is probably more than you wanted to know but just based on my experience, use GL-4 if the transfer case or transmission calls for it because it does in fact seem to eat brass!
Unless specifically called for, I would not use GL-5. But, I would look at any tranny that was giving any gear change issues and try Redline MTL first. It has magically "fixed" more manual tranny's than I can count on both hands. It'll handle all the loads you care to throw at it and it will not hurt synchros
It's spendy, but well worth it
As far as transfer case goes, I'd be using GL-4 as I see no reason for GL-5. If it has a push button engagement (motor driven shift), I might use Redline MTL to make it easier to engage in cold weather ...
And, if you want to keep differentials alive with manual tranny's and big load, you use Redline Shockproof. It has kept about a zillion differentials alive in drag racing with launches under big power with very sticky tires for a long time. It is far beyond GL-5