I'm having trouble finding transfer case fluid and instructions that are consistent. I think it's because mine is still fairly new, and apparently, things have changed with these cars lately. Here's one explanation:
My two cents: I have a 2017 Clubman ALL4 with the same system as a Countryman ALL4 and have serviced all the driveline fluids.
Front “transfer box”/PTO and rear differential gear units are pretty simple “fill until overflowing” things. Front has a drain plug, but the rear only has a fill plug so you have to suck it out.
I use AMSOIL Severe Gear 75w-90 in the front transfer box/PTO and Mopar synthetic 70w-80 GL-5 in place of the insanely expensive BMW “G3” 70w-80 GL-5 for the rear differential.
But…. The official BMW TIS procedure for changing the “Haldex” (genericized) / HOC fluid is crazy.
You’re supposed to remove the exhaust, unbolt the driveshaft from the rear drive unit, attach ratchet straps to the inner CV axle bodies and loop over the rear subframe. Then unbolt the entire rear drive unit and rotate the forward end of the rear drive unit 90 degrees.
Then unbolt the Haldex unit (4 bolts) and remove. Drain Haldex unit and rotate/tumble the unit 360 degrees by-hand to get the fluid out of all the chambers inside. Reinstall Haldex unit and leave pointing downward.
Then refill unit with 520ml (+/- 50ml) of proper Haldex/HOC fluid at least at “room temperature” / 18C or warmer. It’s correct to say DO NOT fill until fluid runs out the fill hole.
“The oil filler plug on the reservoir cap is not a fill level control screw!” - BMW TIS
The final step says:
“Perform hang-on clutch oil bleeding routine.
Service function -> Drivetrain -> Longitudinal torque distribution -> Bleed
60 seconds waiting period
Repeat service function “bleeding routine”
———————
Or what I did was carefully suck fluid out the fill hole and get a good measurement on it. My notes say I got about 250ml doing that. Then refill with the exact same amount. I used Ravenol “AWD-H” Haldex-specific fluid. Done two winters in a very high snow/ice area and my AWD still works perfectly. About to do it again soon.
The VW guys say they’re supposed to do a bleeding routine too on their very-similar system. Sounds like most people just cycle the key a bunch of times and the pump will run/bleed itself through its normal startup cycles
I’m kicking myself for not checking the fluid level vs. the fill plug when I first did this. I’m kinda wondering if the “correct” fluid volume is so close to the fill plug that it wouldn’t even matter if you filled until it overflowed. That’s what the VW cars do