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Changed front and rear differential oils today, with 68,000 miles. Mercedes does not specify differential fluid change for the base 4MATIC on W166. The hardest part of the job was wrestling with the front underbelly shield. It's wedged/underlapped behind both the front bumper fascia and the rear belly pan. The rear 2 screws need to be started, then the shield needs to be bowed and slipped behind the bumper fascia, and the front two fasteners started. Then all fasteners can be run down tight after the shield is in place.
Front axle: oil was clear and gold in color; it looked new. The fill plug was tough to get open because it is above the static oil level making the breakaway torque higher, and it is close to the engine oil pan which means a universal swivel was needed. The drain plug was fairly easy to remove. Fill was straightforward. Note to self: small rust bubbles were visible on the engine oil pan, a new oil pan would make sense at 120,000 miles. The oil pan is easy to remove because all fasteners are accessible and the oil pan can drop straight down.
Rear axle: the iron axle housing and apparently lower quality plugs required alot more torque to remove. The outer edges of the plugs were corroded, I'm glad I did this job. With higher mileage the plugs would be very difficult to remove. Acess to the drain plug is tight because of the exhaust pipe. I used a hex 14 bit from Harbor Freight, it was shorter than Husky and Crafttsman hex bits. I was able to easily put the bit into the plug with a wrench, without grinding down the hex bit. The used oil had a slight cloudy/darker color but was still transparent. It's good to have it changed now.
Parts: A3529970032 rear axle plugs, 2 required A1643310056 front axle plugs, 2 required
A001989330312 universal hypoid gear oil, I purchased 3 liters from my local dealer. Another fumbling around experience for them to find the right oil. I told them which oil I needed.
Oil spec is confusing as usual with Mercedes. WIS shows one thing, Bevo shows another, EPC lists all of the numbers, and the dealers have no clue. I went with Bevo, because it seems most updated. 239.71 and 239.72 have the same part numbers, and are specified for W166 non-AMG. However the dealer didn't recognize the part number, so I went with the universal Mercedes 75W-85 gear oil. I have confidence in this because there are no friction discs in the standard 4MATIC differentials.
In addition to the pictured tools, I used a Motive power fill to dispense the oil into the axles.
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
