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oil viscosity

itsabronco

Sr. Member
Joined
Jul 27, 2009
Messages
551
Loc.
Banning, CA.
Been out of it for quite awhile, probably for about 6 or 7 years ! but, what gear oil viscosity goes in the T-case? It is the straight shift. is that a 205? 72 bronco sport. Also what about the oil in the trans? Its a 4 spd but not the granny 4 spd. How do i find out what the trans is? Used to be 3 on the tree. PO put the 4 in it. Thanks !!!
 

TN1776

Bronco Guru
Joined
Oct 24, 2006
Messages
2,632
Assuming a stock transfer case you have a Dana 20. Sounds like you might have a toploader 4-speed if it doesn't have a granny gear.

I run 75w-90 in both diffs, the tranny and the transfer case. That is pretty much the standard for these old rigs.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,355
Yep, what he said. I use 75/90 synthetic in mine. I like the way it flows. Easy to pump in through the small holes and should protect well. I know for sure they both shift like butta'. Especially the transfer case.

The original specification called for 50 wt NON-DETERGENT motor oil in the Dana 20 transfer case. Which, according to some fairly knowledgeable types here, is roughly the same effective viscosity as an 80 or 90 wt gear lube. I have always found that hard to believe, but, I'm no viscosity expert either.
What that oil equates to these days, if you want to stick to spec literally, is pretty much limited to a 50wt "racing" oil such as Valvoline or whatever you can find.

I don't know this for a fact, but I believe the factory recommendation was due to additives in detergent oils and then-modern GL-3 (and also GL-4 I think?) gear lubes having properties that ate away at the bronze bushings and shims in the transfer case.

More modern formulations don't have that issue, so I use the 75w/90 synthetic in all four gear boxes.

Original spec for the 3 speed manual was straight 90 weight I believe. But then, they may not even have had the multi-vis versions fully tested at that point. Who knows what they'd specify today.
Your four-speed, if a Ford Top-Loader, probably used the same lube.

Paul
 
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