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I noticed the flywheel inspection cover on my TL had some black sludge building up on the outside. This is what it looks like inside with the cover removed. It looks like oil mixed with clutch friction material to form this black metallic sludge. I'm assuming it's the rear main seal leaking but I can't tell for sure if its oil or MTF. It's only on the bottom of the bellhousing... the flywheel, clutch, and upper part of the bellhousing (engine side) are completely dry. Clutch is not slipping. Oil level is normal and doesn't appear to dropping.
Depending on the mileage, some seepage at the rear main seal, is to be expected. I think the key to your concern, is in your statement, (" the flywheel, clutch, and upper part of the bellhousing (engine side) are completely dry. Clutch is not slipping. Oil level is normal and doesn't appear to droping") From this statement, IMO, I don't think you have cause to be concerned at this time.