So my GL550 has recently crossed the 100k miles mark. I did a whole bunch of preventative maintenance last week, but ran out of time to do the transfer, front and rear diffs oil change. MB said they are lifetime oil, but I want to change them any way.
The front diff and transfer case both use H8 for both the drain and fill bolts. And even at 100k, the front diff and transfer case oil were still clear enough to see thru.
The rear diff uses H14 for the drain and fill bolts. These bolts take some creativity to get to. I used a 14mm wrench to turn the H14 socket, the exhaust pipes are in the way of getting the socket in there. And the oil definitely needed to change, it was blacked.
All and all it was a very easy job, I did it in my driveway with the vehicle raised. Would be a whole lot easier if I have a lift. The vehicle drives a lot smoother now, not sure if it’s real or mental. All three units used right around 3 quarts of gear oil. Front a little less than 1 quart, transfer case right around 1 quart, and rear diff a little more than a quart. Anyway, hope this will help someone.