My original transfer case was replaced with a different one as part of a recall in the first year at around 5,000 km. Since I had the same questions regarding the oil change procedure in conjunction with the oil cooler, I checked the oil level in the transfer case at around 18,000 km. To my surprise, about 700 ml of oil leaked out of the transmission after opening the check/fill plug, which shouldn't have happened. I then drained all the oil, and another 2 liters came out, which together corresponded to the total fill capacity. Since this amount was pointless, as it would have been impossible to measure the oil level using the overflow method, I pumped the 2.7 liters back into the transfer case. This can be done with a pump. You simply attach a suitable nozzle to the fill opening on the transfer case and pump in 2.6 liters. Then quickly cover the opening with your thumb and close it again.
After driving approximately 13,000 km with an overfilled transfer case, I discussed this issue with my dealer. Suddenly, they were talking about a total oil capacity of over 3 liters. However, no one could provide me with proper instructions on how to change the oil in the transfer case and the cooler. I then calculated how much oil could be in such a cooler and in the lines and arrived at just under 600 ml. Together with the 2.1 liters in the transmission, this brings the total oil capacity to the specified 2.64 liters.
Based on this finding, I decided that the dealer had made a mistake when changing the transfer case and had overfilled the transmission, as they didn't have any reliable data at the time.
I drained the approximately 2.7 liters of oil from the transfer case and refilled it with new oil up to the overflow at the filler opening, which corresponded to the required 2.1 liters. I've now driven approximately 27,000 km and checked the oil level in the transfer case several times. It has remained at the level of the filler plug ever since. I haven't had to top it up.
I haven't yet found a good way to change the oil in the transfer case and the oil cooler. I'll simply change the oil in the transfer case more frequently, which will keep the total amount of fresh oil in the system high. Of course, I could also remove the hoses from the oil cooler, but that usually makes a mess. It's easier to just spend €20 more often for 2 liters of transmission fluid.
