Working on my 83 300CD.
Just did the valve adjustment, and timing chain stretch measurement (2.5 degrees ) last night. The valve cover was messy and I was tired but I reached deeper and started cleaning all that black crap off inside & out. It was on the third rinse/wipe that I realised what I was seeing. Some PO had smeared black silicone seal along the edges of the tin oil deflector (if that is what it is)inside the valve cover. It had started to peel off .
Years ago a buddy assembled his oil pump on his just rebuilt Harley engine with this stuff. It went about 20 or 30 miles before the silicone pealed off inside the pump plugged up oil galleries and destroyed his new engine
So a lot of work rubbing with a cloth and picking stringy blobs with a haemostat, and clean now, I'll put it back together this aft.
Clean is good! If you clean everything before during and after, you always know more than you would otherwise. Keeping the old Harleys tuned, clean and shiny is when we used to learn about loose bolts, spokes, iffy wiring, etc etc. I think that's why Clean Machines run better
Alberta Luthier