dwcasey,

The fill, and level checking plug is on the side of the differential (the right side on my 190E and 240D) and it is a 14 mm hex socket.

In a level area, without ramps or some way to elevate the car, you will be hard pressed to get the space needed to remove the plug and add fluid, then reinstall the plug, but it can be done. I have done this by putting the car up on a jack and jack stands, removing the fill plug, then the drain plug, draining the stuff out, putting the drain plug back in, and filling the differential. I put the fill plug back in kind of loose, and then lowered the car back onto the level area. I pulled the fill plug back out and let the excess run out, then put the plug back in and tightened it. The fill plug, and I believe the drain plug, are tapered pipe threaded plugs. Both take the 14 mm Allen wrench, which I believe is available from Sears. I got mine from Snap-On about twenty five years ago, but I think Sears has them now.

I loosen the plug by tapping on the "handle" of the Allen wrench in the openning direction with a hammer or mallet, then turn it by hand. I tighten them using the same method. I tighten to the best I can by hand, then tap the handle to get the last flat or so with the hammer.

If the fill plug comes out and no oil leaks out, you need to add some. If you put your finger in and can not feel any oil, you probably need enough that it is worth just draining and refilling with new stuff.

I like the Redline High Shock, heavy weight gear oil. You do not have a limited slip differential so you can likely use any suitable heavy gear oil. I prefer the Redline stuff because it is fully synthetic and the additive package for the high shock stuff is especially suited to reducing noise from the rubbing contact between the gears.

I am not associated with Redline, just a happy customer. It is expensive and you have to go to their website to find a local distributor. In my area a big snowmobile dealer is the closest dealer and he has a full line of their products. Good Luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)