Rear diff fluid weight rec
 

I've gone through the threads on rear diff fluid but most seem to be comparing brands. Curious if anyone has compared weights?
In Colorado so cold and warm weather DD, but also track it a few times a year in warm weather.
Anyone have any recs on stock 75W-85 vs 75W-90 vs 75W-140 for those conditions?
Thanks!

I live in WA so similar various hot cold weather.
I use TriboDyn 75w90 Synthetic Limited Slip Gear Oil
With ebay magic alluminum rear diff cover (100% identical cusco knockoff for 1/5th the price) adds fluid capacity and cooling fins.
Don't have any way of measuring temp but so far zero issues, leaks, or noises.

<preparing for the barrage of "did he really ask that?> Why bother changing the diff fluid at all? I understand there's a fill and drain port, but unless the diff is being swapped out or overhauled because of indirect damage, I didn't think diff fluid was one of those things you "just change out every 30,000 miles." Thought it was for the life of the car. It's a closed system. Even if metal shavings are coming off from standard wear, I haven't heard of these things cannibalizing because the fluid wasn't changed. If anything, I've heard people doing it wrong (clogging check valves with shavings that wouldn't normally interface unless being flushed incorrectly).


I'll have to go back and check the owners manual, but this is a topic I fought a Honda dealer on. They said "Honda recommends changing it minimum of 30,000 miles." I asked "where in the owners manual does it say mandatory?" Far as I know it doesn't.

Interesting that screenshot doesn't have a lot of the Notes. Mine has Rear Diff with a Severe Driving note to replace every 15,000mi. (And brake/clutch fluid replace every 15,000 in high humidity or mountainous terrain.)

That is interesting, I wonder if they adjusted the maintenance schedule..?

I have a mechanical LSD and it's supposed to be changed every 15,000 miles, but I was under the impression the stock Torsen could go 60,000 like that screenshot showed..

15k is for "severe" only. Figure I got a cheap car I can care for well with some light track abuse so will probably follow the severe sked.


I put some Amsoil 75W-90 in diff and MT this week. Comments here on Amsoil thread.


Noticed the Amsoil recommends 50k or manuf, whichever higher...

Diff fluid should certainly be changed (or at least inspected) somewhat regularly.

The whole 'nah don't worry about it' in post #3 is concerning... but I digress.

Gear fluid is an oil/lubricant, just like engine oil or ATF. These fluids have certain lifespans, mostly dependent on temperature.
Additionally, debris is an issue. Engine oil and ATF have circulation (pumps) with filters, the diff (and M/T) do not. The best they have is a metallic drain plug.
And I guarantee you, when you pull that drain plug out, it will have a thick sludge of metal goop stuck to it and you will feel very guilty you didn't swap the fluid out 15k miles ago.

Another secondary reason is to maintain fluid level. Leaks happen sometimes. Sucks to find out about a small leak when your diff blows up or starts howling. "Oh...there's some residue here...I see."

As for fluid weight - I've never heard of the xxW-140 used in anything except heavy-duty truck applications. It's thicker, so it has more room to heat up.

My preference would be to stick to the recommended weight (75w-85/75w-90 have been the standard for years) and get the best brand in that weight.
As far as M/Ts go (manual specifies same fluid for diff and M/T), mine is already noticeably notchy when cold, at only 53k miles. But, to be fair, it's like 34F out here in the mornings.