I don't know of any engine that can not take syn. As for which weight, use something like 5W-40 or 0W-40. Since you plan on using Redline, I don't know what weights they have. 0W-40 is the factory fill for new Mercedes.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
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