My 89 300te had the oil slick on the belly of car, from the passenger side of the engine, along the transmission, and all the way down the floorpan. It was leaking from the front-cover-at-top (the infamous U seal), so I replaced it. But it was also leaking from behind #6 plug, which I didn't realize until I found the oil kept leaking. Only happened when the engine was running and tough to notice. If you prop the hood all the way up (vertical) and shine a light behind the back of the head, so you can see where the engine connects to the tranny you might be able to see where the oil is coming from. A dentist's mirror might help. You can also shove a rag back in there and pull it out. Even a small amount of fresh oil on the rag indicates a leak. I noticed that this leak would also drip onto the exhaust and smoke. New head gasket fixed that.

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89 300te 222,222
92 300e 190,000