More Amsoil Black Gear Oil

I would use an infra red temp gun on the diff if the OP thinks it’s heat that turned his Amsoil fluid black. The early colour change could also have caused by a coating of black heat affected fluid inside the axle tubes left over from when the first diff failed.

This a clever idea.
How long have you dealt with this shop? Did you meet the mechanic?

Do they stand behind their work?
I’ve never dealt with ECG before. I’ve got to get more info. Is there a good lab you recommend that checks gear oil? What do I check for?
Let me add this that may affect your advice.

ECG supply did not replace the seals when they replaced the rear diff. It ended up leaking on the backing plates on both sides. (I did check the fill hole and it was still good and full.) That is why my mechanic replaced everything from brake shoes to the rear diff. The leak had ruined my brake shoes.
Do you have anything added on under that truck that would be diverting airflow from keeping the diff cool? Some of weird skid plate or something?
@Gebo AMSOIL Severe Gear contains a good amount of alkylated naphthalene. That is a base oil that also has extremely good cleaning properties. AMSOIL has an industrial facility for blending and packaging their lubricants. Because of how such facilities operate, it is impossible to get a bad batch. To put it into perspective, you have a greater chance of winning the Power Ball or getting struck by lightning twice than getting a "bad" batch of AMSOIL gear oil.

This is not going to be cheap, but in the grand scheme of things it is going to be cheaper than taking your diff apart again or arguing with the shop. Get some more AMSOIL in your favorite viscosity. Change it twice every 1000 to 2000 miles. If the it keeps coming out black then you have an issue. If it starts brightening then it just cleaned the dirt left by the diff that went bad.

AMSOIL Severe Gear is one of the best products that AMSOIL makes. For example I'm not a fan of their ATF, but their gear oil is really good. It uses a proprietary formulation with the base oils borrowed from their wind turbine oils and the additive package is made by Lubrizol. It's a modern low Phosphorus-Sulfur additive package.

Or you can make more assumptions and try this, try that, without any results. The choice is yours.
Some of you may be aware of my rear end saga on my 2002 4Runner. "Apparently", one mechanic ago was a little shy on my gear oil
and it burnt up my rear differential. It came out black. Got a new rear diff from ECG supply and had everything from the brake shoes from one side to the other side replaced. Diff from ECG and all the other were Toyota OEM parts. About a $3000 job in total.

I got it back 500 miles ago and just for kicks decided to check my fluid this afternoon. Went on and drained it (Amsoil 75W-110) with about
1000 miles on it an d it is black as tar. Looks like you poured graphite in it. I drained out 2.5 qts.

Did I get a bad batch of gear oil? Thank goodness I saved a sample this time. There is no way this is a low fluid situation. I called my "new"
mechanic and he's thinking I have a gear oil issue. Is Amsoil supposed to turn black?

I'm an Amsoil fan boy and this is not a slam. It seems to me I have a problem.
I have run 75w-90 Amsoil Severe Gear in everything I've owned for 15+ yrs. It's never been black when changed. Recently I changed/drained the 4.5 gals out of our motorhome differential housing and did an analysis.

Here's the thread I did:
There’s a brilliant man on here I have been pm’ing. He believes the Amsoil is just cleaning out the residue from the initial mess up.

He, like most of you, suggests doing a few more changes every 1000 k so or and see what happens.

Also, this is the rear diff that had the rear plate replaced and welded back on before the rear diff went bad.
Find the problem in your differential. It's not the gear oil. I am with Pablo and others on this.
I agree. I am not bashing a product I have been loyal to for 35 years. It just scared me that I have seen this 2 times in 4 months. And I just paid $3k to fix it. Yikes!!!!
How long is their warranty?

Reinstall the Lucas product they recommend and keep driving. If there is truly an issue, the diff should grenade pretty quickly.
I've had my fair share of diff issues- due to GM guv-loc replacement with a Trutrtac. Friend/mechanic did the job and set it up wrong- so cost me again for labour and bearings. Two choices- find a guy who REAlly knows what he's doing (as I did with no issues since) or find a unmolested oe diff at the breaker. There's no magic oil that will crutch a defective diff.
I got my sample saved just in case....


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I'm gonna do some flushes to see if it clears up. I was told it is probably just the crud that was in the axle tubes.