Brother of The Benz, mikef
My 1987 300E with 179,500 miles has had the diferential gear lube changed once a year since new.
No gear noise or seal leaks.
Red Line Products are the best on the market. If I had started with that oil I would still be using it.
But I didn't so here is the way I do mine.
I use gear lube 90w and add 8 ounces of gear-moly to one gallon of gear oil(grease in some groups).
To remove the plugs using special allen wrenches, Auto-Zone has a 1/2" sq.drive 3 socket set, H12,H14,H17 sizes for about $7.00 the set.
Ofcourse you would remove the plugs in the proper sequence.
After you've done it the first time you too will do it every year.
Happy Trails Beep Beep from The Spiderman in Houston!!!