I agree with Clauser1, there is something wrong beyond which oil you are using. It would be better in the long run to get it fixed. Also, there may be problems changing to synthetic in an older engine. The main reason for this that over the years regular oil can leave a film or varnish over the more important seals and gaskets. This film acts as a seal itself. Synthetic doesn't do this and cqan wear that varnish away, revealing oil leaks that the varnish was preventing. If you really want to clean your eninge, find the problem, get it fixed and then try an engine cleaner. I am pretty sure that both Molly oils and Redline have an additive you put in your crankcase a few hundred miles ahead of an oil change. Drive around while this cleans your engine, and loosens the sludge and alot of the other garbage. Then drain the oil (maybe even overnight as Larry Bible does - this gives the sludge more time to ooze out). That should have you fixed up pretty good.
1995 E420 - retired
1986 420SEL - retired