Have you tried Free-all? It's suppose to be one of the best. In my experience, it is definitely better than Liquid Wrench and PB Blaster. The oil cooler has steel nut on aluminum, not a good combo. I had to work the nut back and forth 2 to 3 degrees at a time with the 2 wrenches, took about half an hour of that to get the nut off. If I didn't do that, I have no doubt it would have stripped the oil cooler's aluminum threads.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked