Is my Differential Fu#*ed?

[BayAreaE93]

So ever since I bought the car two months ago, I have had a major groaning when making sharp turns at low speeds when cold. I did some research and found that it is actually commonplace among these E9x M3s, and BMW NA came out with their own 75-140 formulation with extra slip additives to kill the groaning.

Having done even more research, I found that this is not ideal and that while it kills the groaning, it also limits some of the LSD's functionality, which I'm not a fan of.

A few weeks in to ownership, I realized that I was about 250ml low on diff fluid. I topped her off with Castrol Syntrax 75-140. The groaning subsided a tad bit, but was still definitely there. I just figured that being low on diff fluid caused excessive groaning, and everything was fine now that I was topped off, and I had to just live with it, if I didn't wanna use BMW's nasty stuff. Here's where it gets interesting. I had developed a bi-weekly routing of topping off my diff because of a pinion seal leak, topping off about 100ml each time. But during my last top off I mindlessly took out the drain plug instead of the fill plug, and lost about 400 ml of fluid before I sealed her up and topped her off.

And now, the groaning has completely subsided. There is no groan at all. And I'm using Castrol 75-140, not BMWs crap. So this makes me wonder, are the clutch packs in my diff permanently damaged from being run with the old fluid for who knows how long? Are there any real-world driving tests I can do to test complete diff functionality?

FYI I hadn't done a complete diff fluid change because I've been waiting to do that concurrently with the pinion seal job.

[BayAreaE93]

Originally Posted by Mog-10

A real world test is if it leaves two lines of rubber on the ground.

It doesn't and I can still swing the shit I guess that means it's fine?

I'm not too familiar with diffs so I was unsure about how sensitive these things are, especially if they run low on oil for who knows how many miles...

[Helmsman]

OP, similar experience here. With dealership fluid I had a healthy groan over the years at low speed turn, didn't bother me much though.
Changed fluid myself using Castrol syntrax 75-140. Groan completely gone. This fluid should be proper stuff incl only tiny bit of FM so decided Im fine...

[jdamore]

Originally Posted by BayAreaE93

It doesn't and I can still swing the shit I guess that means it's fine?

I'm not too familiar with diffs so I was unsure about how sensitive these things are, especially if they run low on oil for who knows how many miles...

It doesn't? It should. Go try again

[BayAreaE93]

Originally Posted by Helmsman

OP, similar experience here. With dealership fluid I had a healthy groan over the years at low speed turn, didn't bother me much though.
Changed fluid myself using Castrol syntrax 75-140. Groan completely gone. This fluid should be proper stuff incl only tiny bit of FM so decided Im fine...

Yes I believe the Syntrax 75-140 is factory fill and OE spec fluid, BMW just made their extra friction modifier crap for complaining Americans, so the story goes. So far with brand new Syntrax, the differential makes only a tiny a bit of groan when it's cold, so I'm happy. It was a lot worse before, sounded like the tire pulsating/rubbing against the wheel well.

[Bubbles]

Originally Posted by Mog-10

A real world test is if it leaves two lines of rubber on the ground.

Originally Posted by BayAreaE93

It doesn't and I can still swing the shit I guess that means it's fine?

I'm not too familiar with diffs so I was unsure about how sensitive these things are, especially if they run low on oil for who knows how many miles...

Originally Posted by jdamore

It doesn't? It should. Go try again

It should look similar to this