jjpowell, if you switched to synthetic already and if you are not having oil consumption, or leaking issues after 8k miles then keep using full synthetic. as far as oil choice, the most popular choices are Mobil1 0W-40 or German Castrol 0W-30 but anything on the MB 229.1 or 229.5 list will do.

Keep in mind that M1 synthetic turned out to be GroupIII oil, not PAO. So you're paying 2x for M1 and it's identical to the cheaper GIII oils.

Lastly, "dirty" oil doesn't necessarily tell you anything. Ppl always freak at dark oil. Get an oil analysis done if you really want to know the condition of the oil after 8k miles

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Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe