You will find the fill plug on the passenger side of the transmission. It will take a 14MM hex key but you don't have much room to get one in there. There is a special tool but it is expensive.

If you want to scrimp on bucks, get a 14MM Allen wrench and cut off a length that will fit in the plug hole with enough sticking out to get a box end wrench onto. That will get it out. ALWAYS take out the fill plug first so that you know that you will be able to refill it after removing the drain plug.

The drain plug is on the bottom and also takes a 14MM hex key.

I have used Mobil One 10W30 in my MB manual transmissions with great success. One of these transmissions has 533,000 miles and still works perfect and has never been apart. The other has 288,000 miles and the same thing.

A couple of quarts should be more than enough.

Good luck,