If it were my car, I would change the oil using a MB oil filter and 0-40W Mobil 1 Synthetic oil. I wouldn't even consider using any type of additive.
Change the oil every 5,000 miles, (or at the FSS intervals if you don't want to spend the extra money), and you shouldn't have any problems.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible