Socal_Tom

So it turns out that the Royal Purple bottle for 75w-90 weight oil is nearly identical to their 75w-140 weight oil.

Yep, I did. I put two bottles of 75w-90 and two bottles of 75w-140 into the non-LSD gearbox.

Before I drain it all out and waste a bunch of money I wanted to ask what you thought about potentially just using it? I'm in SoCal so not like the cold is going to be an issue, it would be the heat I'd think required some thinking.

polecat702

I'd be on the phone to Customer Service at Royal Purple, tomorrow.

Personally, I don't think it'll be a problem.

Mrmerlin

you will never know what oil you have in the diff except for the color and the smell.
Did you rebuild the LSD unit with new clutches?

Socal_Tom

Thanks guys, appreciate the thoughts.

Stan, my ‘84 doesn’t have an LSD based on how the wheels move off the ground.

hacker-pschorr

I have a feeling they will play it safe and say swap it out. They've got nothing to lose by sticking with the specified fluid, not their money buying 4 more quarts

However.... on that note. Keep in mind the pre 85 manual boxes were originally intended to use Dexron III which has the constancy of water compared to 75w-90 much less 75w-140. I have yet to see anyone produce documentation explaining why the change. Maybe they were seeing differential issues and figured the transmission would be OK with the compromise of thicker fluid?? No idea.

Personally I'd swap out for the correct fluid. These early transmissions are fragile enough already.

hwyrnr

For me I’d swap it out just for the piece of mind. You will always think about it and then you will think your hearing things then you’ll drive your self mad feeling you should change it. I’d play it safe and change it. I know it’s money, but cheaper than a new diff.

Socal_Tom

Thanks gentlemen, I'll call them tomorrow and see what they have to say.

GregBBRD

The mixture won't actually do damage.

The thicker oil may make it more difficult to shift, however they both have the same cold rating.

Leave it in and see how it shifts....what do you have to loose?

And the early transmissions were not made to use Dextron....they only did that with transmissions that were difficult to shift, when cold. Once 75/90 became more common (and the synchros had a chance to break-in), almost all early transmissions that were Dextron filled got changed back to gear oil.

hacker-pschorr

Then why do the 78 and early 79 owners manuals make zero mention of 75w-90 and only say to use Dexron?

Mike Simard shed some light on this topic years ago:
From the owners manual in my 78:

SwayBar

Lots of reasonable replies here, and Erik, nice job with the WSM.

For my 0.02 and being strictly a manual transmission guy, if it were me, I agree with Greg and say run it to see how it feels. If there's no change, keep it in.

On the other hand, if you do change it out, I'd recover it versus dumping it out. Not sure what you could use it for though...

Socal_Tom

Thanks for the discussion. Let me double check with Royal Purple tech line but I'm inclined to try it and see. If I do I'll report back as a test case for the forum.

Not sure, but if I recover it and sell it I'll be sure to list a price.

(Dr Bob and Hacker-Pschorr, see, paying attention. Grin.)

hacker-pschorr