While the high detergency of "synthetic" oils can help clean up ring lands, it's not going to relieve a severe case. It sounds like you may be at the point where a hammer and chisel are needed to clean out years of accumulation, Felix.

I suppose there's not much risk at trying. Your most economical choice will be Rotella's Syn 5W40. Many folks run this oil in their old 5cyl cars with good results. It's a modern all season/all climate oil.

Good luck.

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1998 W210 diesel (wiped out by a texter)
Baum spring compressor "for rent"