222k Mile Transmission advice

[Sundevil]

I have a 2001 X5 3.0 with 220k miles on the odometer. I was talking to the parts guy at the dealership and saying I was thinking of doing a fluid/filter change and he said I would be nuts to mess with the fluid with that many miles. He said I should top it off (some times when it is cold it is slow to engage reverse). I rescued the car with 200k miles on it and I do not know if the fluid has ever been changed. My understanding is originally the GM transmission in the 3.0 used Dexron III and then changed to Dexron VI. My question is, if I was going to top off the fluid and do not know what is in there, which new fluid should I use??

Any advice is appreciated.