what oil to use in the 850 transaxle?

Thank you for confirming my suspicions. Matt seems to be focused on the synchros, but it's important to think about the hypoid diff. I bought the Sta-Lube GL-4 from NAPA (in stock at my local store) since it meets the original criteria from Fiat and is equivalent to the Liqui-Moly I have successfully used the past 20 years. Thanks again for the very useful discussion!
 
I finally picked up my gearbox yesterday. I had one of the local gearbox shops change the pinion and ring gear in one of my spare boxes. This was very labour-intensive as the mechanic had to keep pulling the pinion out to alter the number of shims. The ring and pinion gear is NOS I bought a couple of years ago for the 850 sedan with 12" wheels. New ratio is now 4.63. The shims were roughly 2mm thick! The wear pattern is very nicely set up. I had to order in some GL4 gear oil and settled on this one.
 
fiatfactory said:
as far as EP additives attacking the coating on the sintered steel synchro rings, I've only ever seen that once in many transmissions I've overhauled... and the first thing to remember is all EP additives are not the same. The only time I saw the "green death" on X19/128 synchros was on a trans that had been sitting around for some time, Ive pulled down transmissions that obviously had EP oil in it (it stinks - you CAN;T miss it) and no sign of green death.

To me it must have been the EP additive breaking down and oxidising, or combining with moisture to form yet another compound, which then attacks the metal, remember we are not talking yellow metals here, thats a totally different story.

A lot of EP additives are Sulphur (or Sulfur for those in the US) based compounds, oxidise them and add some H2O and there a good chance your going to get something aciidic in the process... IMO that's what causes the green death on the sintered metal synchro rings as used in a 128 / X19

The coating on that type of synchro is molybdenum, certainly is on Alfa Romeos, and I have some packets of Goetze synchro rings for Fiats that appear to have the exact same type of coating, and even have "Mo" in the part number.

www.sciencedirect.com

Frictional properties and wear of a molybdenum coating and a bronze (Cu-10%Sn) with friction modifier fillers

A new manufacturing process and a new family of composite materials (bronze matrix + friction modifiers) for lubricated tribological systems, such as …
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SteveC
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When I was looking for a suitable transaxle oil I went to a friend who runs an engineering supply shop. I was after a GL4 oil that none of the auto shops bothers to stock now. He asked me if there was any gold/ brass sparkly stuff in the oil that I drained off and there was! So the steel sintering coating on the synchros probably looks like brass as well. Looking at the synchros they look like just steel when looking in the box. The box I am using now has only ever seen 1970's EP 90 and the oil going in should be a pretty good match.
 
Before I bolted the bell housing back on the gearbox today I cleaned out the interior. The sludge was definitely grey. The synthetic oil that I drained out must have made the particles look like brass.
 
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