Over the weekend I did an oil change on my E39 M5, and I also changed the differential oil. This is an important and often overlooked fluid change.
The E39 M5 differential is a limited slip differential, allowing for different amounts of power to be directed to the wheels based on driving conditions. The car had been well maintained by a previous owner, but I don’t know when the differential fluid was last replaced. So I tackled it along with the usual motor oil change. I use Liqui-Moly 10×60 oil and Mahle filters in my car — see this post from last year for more information.
The differential fill and drain plugs are recessed, requiring a special 14mm hex socket. This wasn’t available in any of the chain auto parts stores in my area, so I ordered it from Turner Motorsport. The spacing is very tight between the fill plug and the right side exhaust, but there is just enough space to get a 3/8 rachet in there with the hex socket on it.
The plugs were surprisingly easy to break free. Important rule — always loosen the fill plug first, prior to drain. Otherwise, you might drain and then be unable to put the fresh fluid in. I used Red Line 75W-90 on the recommendation of Mike Miller’s “Old School BMW Maintenance Schedule.” This really useful document is available for the asking at firstname.lastname@example.org.
The differential fluid capacity is 1.2 liters, so you need to buy two liters. I’m glad I had extra, since I wasted some wrestling with an inefficient suction pump. Luckily I also had a siphon pump, which worked much better.
Now the differential is good for at least another 30,000 miles. Hope this information is helpful to anyone planning to handle this easy maintenance step themselves. Happy Fourth of July!
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