Hello again, everyone,

When I removed the drain plugs from the transmission and differential on my 1985 GSL-SE I discovered a layer of what looked like oil sludge.

The gear oil I drained was very dark but still had an amber gloss to it while the "sludge" was jet black and felt very thick (thicker than the amber gear oil I drained but not as thick as grease).

Here is a photo of the "sludge" on the differicial drain plug:

Here is a photo of the "sludge" on the transmission drain plug:

I can feel a shallow layer of this thicker, black oil sitting at the bottom of the diff and trans.

Is this dark, thick "sludge" a cause for concern?

I'm thinking about pulling the diff and trans apart to clean them out and replace the seals while I am at it.

Thanks in advance.

-Griffin

 

Totally normal on both old and new cars, the plugs are magnetic and that sludge is actually finely ground metal particles soaked with oil. All gear boxes, engines, basically all complicated mechanical metal things will produce this, but it is always far more pronounced on transmissions and differentials since they have far longer oil change intervals and they don't have and oil filter (the magnet is serving as a kind of pseudo filter by removing free floating particles from the system). Don't pull anything apart, you would be shocked how clean they are inside if you did. Also The dark color found in used gear box oils is typically a combination of oxidation of the oil and the by product of wear items with in the gear boxes, manual trans syncros and automatic trans bands shed a tiny bit of material with each gear change, as do clutch packs in limited slip rear diffs, again completely normal. As for as the the shallow layer you feel sitting at the bottom of the trans and diff trust me, it's not going anywhere, but if you're really worried about it you can throw some cheap gear oil in there with about %25/vol auto trans fluid and drive it around a bit enough to get the gear boxes to operating temp and then replace the oil again. The last tip I remember reading on this forum years ago and have never felt it necessary to do myself so take it with a grain of salt, otherwise you could pump a bunch of atf through the gear boxes just to flush anything lose out. Sorry for the loooong reply, I managed an oil chance place for about 8 years so I've seen this situation a lot, really you're not in trouble unless you find large shards of metal on the plugs like I did when I bought my first rx7 (pictured below).

 

Yep, normal and no cause for concern. Just put in new oil and change every couple years.

 

Make sure the Breather on the Diff is free and working..otherwise you can get a pressure build up from heat.,and that can cause all sorts of issues.
I can see that happening with the oil.