[plain OM]

I noted a reference to "magnets" (plural) so I presume the car is AT, with two differentials, front and rear. If that's the case, was the fluid "black" in both, or only one, and if the latter, which? If the rear, does the car have a LSD?

I ask because I've been changing the front and rear differentials on my 07 annually, and have noted that the front fluid drains with a honey color similar to when new. However, the rear always comes out noticeably darker. I've been attributing the darkness to the fact that I have a LSD, and the dark material is from the rubber seals between the differential case and the viscous-coupler's plates and fluid. Perhaps I'm wrong; some comparisons might be helpful, and might help explain steved's observations.

 

[plain OM]

Plain - I would also suspect more debri finding its way to the breather on the rear diff vs the front which could also contribute to this.

Yes, very possible.

Because the vent is just at the top of the rear cover, it's going to be in the air flow around the differential, with it's high level of road dust etc. However, in order for the dust (or whatever it is) to get inside, there would have to be a pressure difference between the interior of the differential and the outside. (The vent tube is capped.) I wonder if this would be the case. There might be some air transfer due to heating and cooling, but would this be sufficient? Perhaps.

If this is a factor, wouldn't pretty well all rear differentials exhibit noticeably darker-colored gear oil when drained, which is what steved reported? (What do you do to have golden colored oil drain out after 80k?)

 

[plain OM]

Keep in mind, RP starts out dark purple but this (while still somewhat clear on draining) was black in the pan (and you could see particles entrained in the fluid).

I misunderstood -- I thought the oil was dark each time it had been changed, but on re-reading the text, I now appreciate that the "black" was only in reference to the used RP, which, as noted, does start out darker.

I have a number of clear, calibrated, plastic measuring cups (dollar store) that I use to examine the color, and compare the oil drained from the two differentials as well as new fluid each time the fluid is changed.

I'll be changing the differentials again in a few weeks and will try to take (and post) some photos that show the differences. Perhaps we're using different terms (dark, black) for essentially the same thing.

 

[plain OM]

. . at ~110-115k, neither were anywhere near black. . . .

Does your 04 LLB have a LS rear differential, or is it a regular, open, type?

 

[plain OM]

"stuff" in it, which didn't seem to affect the wear; but it was definitely there and not normal.

I would tend to focus more on the particles rather than the color -- whether the particles are soft, e.g. some sort of gum formed by the oil (probably not of great concern, but suggests more frequent changes) or hard, metallic pieces. If metallic, size would be the next parameter. It's not unusual for very tiny dust-size material to be found -- the gears do mesh and wear, as is evident from the patterns on the teeth even in a properly running differential -- and so some metal material might be evident.

One sort of test I've done is to strain the oil through a paper coffee filter. The tiny wear particles will darken the paper, but won't be individually visible, whereas larger "chunks" will.

I would wonder if the particles were oxidation remnants?

Are you referring to oxidation of the RP, or the metals? I suspect the RP. (No reason for the metals to oxidize/rust unless the car was parked for a long time or water was collecting inside.) If so, then perhaps it's not up to the task, or at least the oil change interval, assuming there weren't any unusual situations, such as frequent spinning of one wheel.

All speculation at this point. I suppose the definitive answer might come from having the fluid analyzed by a lab (e.g. Blackstone).