I owned, and restored, a 1980 400i. The engine should be dead smooth and as I recall, very quiet. It was definitely the largest and most complicated engine I've ever done. Working on it is not so easy because of all the stuff crammed into the engine bay. Two complete K-Jetronic fuel injection systems, two alternators, two oil filters, two of almost everything except one crankshaft. Very nice cruiser, though.
The crankcase holds, I believe, 17 quarts of oil. I did not use synthetic oil, given the car is not exactly a hotrod. I did install platinum plugs, and the original wire set was still good. The distributor cap on mine was cracked into three pieces - that was an expensive part. Did you replace both fuel filters?
Have fun and enjoy that V-12. Is it an automatic, or manual?
Steve