AMX1397, I don't doubt thats what the dealer told you. They are always going to say MB products only. It's a liability thing.

I also agree that there are some things that requires VERY specific lubrication... like auto transmissions due clutch properties, valves, etc... they have very precise friction additives.

But when it comes to a differential, unless it's an LSD (which we all know we dont have, except that dealer apparently LOL)... gears are gears.

Per the manual page 267 (SLK):
Recommended fluid - "Hypoid Gear Oil SAE 90 or 85W-90"

I researched this and read a LOT before I changed mine to Mobil 1 75W-90. Believe me there are a TON of threads that show most go to 75W-90. Everything I have read points to it being just as good if not better...

As usual, just my opinion.

FWIW, I'm a 37 yr old - 19 yr military aircraft mechanic that has worked on cars with my dad since I was old enough to hold tools. Not that that means a ton, just want to clarify that: 1) I'm not some 'kid' that believes everything on the web and 2) I am not a certified car mechanic... lol

Bottom line, do what you feel comfortable with... there are a LOT of opinions on the forums.