I have used fully synthetic on my 450 which has hydraulic valve lifters. I am not sure on what should be used, but I think that todays better oils are fine and so far no problems nor leaks. I also only drive about 5000 km a year.

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MB 350 SL 2.8 -72
MB E220 -95
MB 230 E -82
MB 450 SEL -80 (project)