I'm a believer in frequent oil changes, perhaps more often than necessary but I figure it's cheap insurance. I've change oil at 3,000 miles in all my cars, including Castrol Syntech 5W-50, in my motor home (Ford 460 engine), which I sold recently. Castrol is the only synthetic I've used, but since Mercedes and all the owner's I've seen post messages recommend and use Mobil 1, that's what I'll use in the 420. In fact, I may go ahead and switch soon. I've just owned this car for about 700 miles, and had the oil changed during the pre-purchase inspection. The mechanic said, however, that the drained oil was quite dirty, so I'll consider that oil change as a "flushing."
I'm also wonder if Mercedes has redesigned or upgraded the chain tenstioner since the original production. In another words, is the new tensioner, I have, "better designed" than the old one. Also, whether synthetic oil will improve the performance of the tensioner. I guess those are questions without easy answers. It's doubtful that the ratcheting-type tensioner is adaptable to older engines. The best advice given on these posts is to check the chain stretch about every 30K miles.