Preface: I've been running a synthetic 5w40 (Schaeffer's) in my car for the last year/7,000? miles, and it showed signs of leaking when I got it but doesn't leave any drips. I've also done multiple oil changes on 3 other 617's with Rotella T6 or Schaeffer's 5/40 and none have started gushing like some swear they're bound to do.

No downside to 5w40 unless your engine is in poor enough shape that the dirt helping the seals do their job gets washed out by the better cleaning properties of synthetic. And for a car driven regularly at all, in anything but all-city driving, intervals can be safely extended from 3K to 5K, or further depending on oil analysis results.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".