I'm going to be changing out the rear differential fluid and I'm looking for recommendations on type of fluid. Is synthetic a good choice? Anything else to watch for?

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1989 300CE

I would go with Synthetic since this is an item that does not generally leak and is a relatively high change interval. I went with Synthetic Mobil1 85W90 on both mine and the rear diff is quiet, no noise.
Nothing much to watch for unless the fluid hasn't been changed for 150k miles or has leaked out....just make sure to drain into a clean catch and check for any metal in the fluid.

Undo the top one first, then bottom. Make sure to 'crack' the bolts not tug on them as you can strip them...not something you want. Drain and run through some of the new fluid to semi-flush any leftovers out until it comes out clear. Replace bottom drain plug and fill until it starts to come out of the top then put top back on...done!

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

How often should the fluid be changed?

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Smoke Silver 1994 E220 Coupe Automatic
0 kilometers 6 September 1994
100,635 kilometers - 6 September 2010

Very easy the special wrench is very handy.

I recommend every 100 000km, including gear box.

I use always Mercedes fluids specifications are matched to their hardware.

good luck

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Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

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Son's one: GLK class, 220CDI, 2009, W204

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Sold: E-class, 320CDI, W210, 2000, 225 000km

I agree. I changed the differential fluid on my 400E at 100,000 and used Mobil 1 synthetic. I never had an issue with the differential over the next 156,000 miles. Maybe the dealer has a better fluid, but it would be hard to imagine you could beat Mobil 1. You will have to get either a 17 mm hex key or hex socket to get the filler plug out.

I think I will change mine, now that you raised the issue.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

What is the fill capacity? Is the Mobil fairly easy to find?

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1989 300CE

Fill until it comes out the top plug. And yes.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Here's an interesting link discussing that the car need not be level for a proper reading of the fluid. I plan to raise only the rear of the car as Loubapache advocates in his post. Does anyone disagree--that it doesn't make much difference whether the car is level to get a correct fill?

http://www.peachparts.com/shopforum/showthread.php?t=29081&highlight=differential+fluid+capacity

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Most of the stuff I've read about over filling or the dangers of over filling a gear box or a differential are related to excess foaming of the lubricating oil. If the original design says to fill it on the level using the fill to spill method then I'd stick with that.

Most differentials have a breather pipe on the top that often gets clogged - if this is clogged up and you've over filled the diff then there is a possibility that if the oil expands and / or foams excess pressure builds up - this could cause seals to fail.

My advice is to not think too much about it. Do what the manual says. Don't over fill. You've just put in some lovely new life blood (oil) - you've done the best you can.

If I had a lift, it wouldn't be an issue. But supporting a car on 4 jack stands and getting under it to break loose two large plugs that have probably never been loosened over a 17-year period is something to think about.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

No, if you use the correct tool, those plugs will come right out. That said, don't forget to remove the FILL PLUG FIRST! That way, if you can't get it out for some reason, you're not in the situation of no fluid and no way to put any in.

Don't worry about working at height - on axles stands - or on a lift. Do it on the floor:-

1) Get yourself a 14mm allen key with a length of tubing (I used water pipe) or a 14mm allen socket and a T bar (you'll could bust a ratchet)

2) Put the allen key / socket into the fill or drain plug.

3) Crawl back out from under the car and lie on your back - scoot back under head at the back of the car feet going towards the front.

4) Put your feet onto the T bar or pipe and put your weight onto it whilst holding your self on your back bumper!

They'll come loose in the end.

As always be careful and work safe - good sturdy work boots are a must.

Good advice from both of you. I will remove the fill plug first and I will try to do it without jacking up the car. It's been so long since I did my 400E diff. oil that I can't remember if I jacked it up or not. All I know is that I have a 17 mm hex wrench and a 17 mm hex socket that I had to buy to use on the plugs.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Autozone told me the fill capacity was 1 1/7 quart. Fill and spill method works best.

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

Don't even attempt to break those 2 plugs without a large breaker bar!!! You WILL strip them if you do. I promise.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k