I had an 02 cooper for 3 years and took it from 53k to 91k with no gearbox problems. It had many other things go wrong but not the 'box. Maybe it had a new one fitted at 52k.
Changed my transmission oil last week on an 05 mcs with the getrag box. Mini say sealed for life. I say balls. Box is quieter & shifts a lot better for £30 of oil. The stuff coming out was as black as night & stank of cat piss, believe it or not.