Auto Fluid or Gear Oil

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I've had pretty good luck with just plain 80-90 so far on mine. I was even bad the first time and probably filled it with rear end oil if I had to guess, but it didn't seem to care much. After I knew about the difference between rear end and standard oil I went a little different direction the second fill. The store was out of plain 80-90, only had gear oil, so I got the thickest plain motor oil I could find (50wt, can't remember if it was 20-50 or something similar) and put two bottles of lucas stabilizer in it to thicken it up. So far it doesn't seem like it's any different than plain gear oil. Took it a little while to mix up good (used to shift hard below 30 odd degrees), but it's pretty consistent now. Just don't try pouring a bottle of lucas down a funnel when it's under 20 degrees outside, literally took over a day to fill...
 
The factory recomends ATF in the service manual but I run 80/90 in all my 4 speeds though, my stuff has some miles on them so I like having gear oil in them. Gear oil is a little less likely to leak past the seals. Some synthetics are a little too slippery for older transmissions, the syncronizers need some friction to work properly.
 
I've run the 85-140 gear oil in mine for over 30 years now without a problem. In a real cold climate, you might want to run something a little thinner but I'd still get something made for hypoid gears so it'll handle the load. Plain engine oil just won't cut it.
 
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