I have come to the opinion, by reading many others', that one shouldn't "change " the ATF, but rather drain and fill equal amounts. It is not easy, nor cheap, to drop the pan and drain all the ATF (which is all synthetic and the same as long as its Mini spec) Toyota Type 4 matches perfectly and for half the price.
When you do a drain-and-fill, you exchange a significant fraction of the ATF, replacing it with fresh clean lubricant, which should make a good difference in the long run, if you do it every 10 to 15k (I do it once a year, or every other oil change).
My reading and my local garage, have told me of transmissions that didn't work correctly after draining all the fluid and replacing all of it at once. The partial drain-and-fill seems to be the ticket. I would say that after 4 years it could benefit from it although up to 30k miles it might not be so crucial.
Good luck, and good for you to inquire about your Mini longevity. Mine is a non S also, so longevity could be our strong suit.

 

That's what most people on the forum mean when referring to a change of transmission fluid in the Aisin. If you do the change three times in succession with short drives in between, you end up with a little more than 80% new fluid since only a portion comes out with every drain.

One good tip I have read is to remove the fill plug first to make sure you'll be able to refill the trans. If it's stuck, or you strip out the head, then you won't be able to drive the car without fluid. It will also make the fluid drain easier.

The temperature method of checking the fluid level is more accurate, and is what is called for in the manual. The fluid temp in the trans should be 100° F when overflowing from the standpipe. Replacing the same amount you drain will only close if the you don't spill any fluid, your trans was filled correctly before the drain, and the fluid temps are the same when measuring.

 

When I said change I meant drain & fill. There is no way to ever drain all the fluid. The torque converter can not be changed.

 

I would change trans fluid by 30K to a full synthetic, at least 3 drain and fill routines and then 2 drain fills every 30 to 40K. Synthetic engine oil and good filter every 5K, your non-turbo should out live us all. When I got my Mini back from dealer with a factory rebuilt R56-S motor it came over filled by a full quart of oil, well over any register marks on the crap factory dip stick. Possibly as a stop gap measure to keep the vacuum pumps alive. I have continued with a full quart over fill for 4 oil changes and counting with No ill affects. I don't think it's possible to changes any of a vehicles vital fluids to often. If anyone thinks it can create a problem, think again, that is a preexisting problem. Dirty fluids don't leak as fast clean fluid does.

 

I'm planning to do a valve body change from Revmax, should be getting it next Wednesday. I hope to get it out of limp mode when the new valve body goes in. Does anyone knows how to get the mini back to 'reset' out of limp mode after a valve body goes in? Revmax said it has to be out of limp mode after installation of the valve body. I'm hoping someone that done a couple of valve body installs can help me on this question.

 

what were the symptoms of a failling valve body? My car slips when changing from 3rd to 4th at every rpm. It feels like the gear goes into neutral, revs up, before dropping the rev and jerks into 4th...

 

I've posted twice here, been a cautious worrier over potential expensive repairs. Now I'm getting a more and more frequent transmission code on my warning display, not the red one, but the yellow, almost weekly now, upon shutting it off from the drive. I think I've felt some unusual downshifts, but otherwise no symptoms yet.
Can my local garage read these codes with their generic computers?
Maybe it's time to drop the pan and look inside...
87,000 miles and partial drain-and-fills since about 50k.

 

I’ve got the same problem on my 2005 Mini Cooper Asin 6 speed transmission. Did you ever get it fixed?
what did you try?