I thought I'd do a little write up on changing the rear diff fluid in the E90s.
This service should be done every 75-100,000 kms.
Special bits you'll need:
Appropriate 75w90 gear oil - Castrol Syntrax Long Life, AMSOIL Severe Gear, Royal Purple Max Gear, Penrite Pro Gear. I recommend AMSOIL but any of these should work fine, the Castrol is the oem oil but very hard to find in retail.
Quantity - If you have a 335i like me you'll need 1.2 litres, all others are 1 litre.
14mm Hex socket
Fluid transfer pump. Due to the lack of a drain plug you will need to suction the old fluid out via the fill plug.
I settled on the AMSOIL after reading a bunch of threads from the USA, it seems like generally great stuff, but anything from above will work.
Take your car for a quick run to warm up the diff fluid, 5-10 mins should be fine.
Since we want this to go as easy as possible we want to warm up the new fluid as well so it will thin out and be easier to pump. I did this by putting the bottles in a bowl of warm/hot water:
Fluid transfer pump I used. It wasn't the best, and I wouldn't really recommend it. Maybe you'd have better luck with something like this or this.
14mm Hex Socket
1/2" Breaker Bar was all I used to get the plug off. It may take a bit of force if it hasn't ever been done before.
You're going to need to get under the car (make sure that however you do it your car is level!), the arrow shows the drain/fill bolt you will be using. Undo it with the breaker bar.
Keep the plug clean if you're going to reuse it. Mine looked fine so I was happy to reuse it. If you need a new plug you can get one here.
Now start sucking as much old fluid as you can out of the fill hole into a measuring jug so that you can keep track of exactly how much you've gotten out. I had a lot of trouble with this step and I would recommend you have a helper to guide the tube around while you pull the old fluid out. In the end I had to fashion up a syringe with some small tubing and a wire so that I could get the last couple hundred ml out of the bottom of the diff.
Here's the old fluid I got out, looks due for changing, had about 100,000kms on it. I got about 950ml out which means I either missed 250ml, or it wasn't there. I couldn't feel, see, or suction out any more fluid. When I took off the fill plug nothing came out, and when you fill it up you're meant to fill it until a small trickle runs out of the fill hole. Either way when I filled it up it took about 1.1 litres so 🤷♂️. This is why it's good to have a measuring jug, you can tell how much should be left and base your calculations on it.
Now you want to fill up the diff with new your new fluid until a small trickle runs out of the fill hole (remember your car must be level), then put the fill plug back on and tighten to 60 Nm (44 ft-lb).
Here's a couple of videos showing the process in case there's anything I've missed here:
Congratulations, you should now have a nicely serviced diff.
In my case due to the disparity between what I was able to get out vs what should have been in there I'm going to drive it for a week and then drain about 800mls out and replace with the leftover new fluid I have, that should flush out as much of the leftover old fluid as possible.