Transfer Case Oil Type? 80w-90?

[MerganserMaster]

I've been driving my Bronco for a bit and have noticed that my transfer case is leaking oil (for some reason, I think the spedometer gear o-ring might be the culprit) and now I figure that its a good time to replace the fluid thats in there (considering its at least a quarter of a century old). My question is if I can use the same Valvoline 80w-90 gear oil that I used in my 3-speed manual gearbox for the transfer case as well? I apologize if this question has already been answered somewhere else but I couldn't seem to find one giving the OK for the usage of this weight of gear oil in a manual transmission transfer case. Also the oil I have states that it is good for GLP-4 or GLP-5 usage so i believe that it does not affect soft metals.

 

[phred]

If I remember correctly the Dana 20 calls for straight 50 weight. You can run the heavier oil but shifting will be stiffer especially in colder months.

 

[MerganserMaster]

If I remember correctly the Dana 20 calls for straight 50 weight. You can run the heavier oil but shifting will be stiffer especially in colder months.

Would 75w-90 be a better choice then maybe? I get to experience winter weather from approximately September to May here in WI

 

[Broncobowsher]

Yes, countless Dana 20s are filled with 75W90. That is the standard gear oil in Dana 20s used in other applications.
For some reason Ford liked running engine oil in transmissions from the dawn of time up through the late 70s.

Fun trivia, 50wt engine oil is the same viscosity as 90wt gear oil.
Engine oil weights are a different scale than gear oil weights. Get into true viscosity measurements (SuS, actual lab measuring that translates into consumer friendly weights) and the windows mostly overlap. Think Celsius and Fahrenheit scales. They both tell temp, they are both in degrees, but the scales are different.

 

[DirtDonk]

I switched to 75W/90 (synthetic in my case) in all four gear boxes on my 71 many years ago.
Front and rear differentials, stock manual transmission, and transfer case.
My Dana 20 always shifted like butter anyway. Better than most. Same with 50 weight motor oil and the synthetic gear lube.

 

[m_m70]

I switched to 75W/90 (synthetic in my case) in all four gear boxes on my 71 many years ago.
Front and rear differentials, stock manual transmission, and transfer case.

Same.......now I only have to buy and carry one type.

 

[Shimmy]

just filled my transmission and TC with Redline MT-90. seems to have quieted things a bit and shifting is smooth