The oil MINI uses in the Getrag 6 speed manual transmission is supposed to be a lifetime fill. I’m not sure what that’s supposed to mean: If you kill the transmission because of bad fluid, that’s the end of its lifetime. A safer bet is to change it yourself every 30-50,000 miles. It’s cheap and easy insurance, and will probably give you smoother shifting as well. The fluid you want to use is Manual Transmission Fluid with the GL-4 specification such as Redline 75W/80W MTF. You’ll need 1.5 quarts for the R53.
Safely place your car on jack-stands with the car level, and make sure you get it high enough to get your bottle of new fluid with pump attached underneath it with enough room to actually pump the fluid. On the R53, the there are two 8mm hex plugs on the side of the transmission across from the PS fan. The lower plus is the drain and the top one is the fill.
If you just want to check the fill, work with the transmission cold and remove just the top plug. You should be able to stick the tip of your little finger into the hole and just touch the fluid. If a little fluid drains out, that’s OK, you either don’t have it quite level or when it was filled it wasn’t level.
If you’re going to change the fluid, it’s a little quicker if the transmission is warm. Make sure you have an oil pan that’s big enough for at least two quarts of fluid. Remove the top plug and then remove the bottom plug. I drained it through cheese cloth and over a magnet to see if any metal shavings came out. I drove the car home from Summit Point in a driving rain storm with no clutch — I fully expected to see bits of my second gear synchro in the drain pan, but was happy to see it was relatively clear and metal shaving free.
Once all of the old fluid has drained, clean the plugs and the threads on the transmission housing. Use a Q-tip or rolled paper towel to get anything that may be left behind. Replace the bottom plug and torque to 32 ft-lbs of torque. Use your fluid oil pump to pump oil into the upper plug opening until it starts to come back out. Clean the threads, install the plug, and torque to 32 ft-lbs of torque.
Go for a test drive and enjoy the silky shifting smoothness you had forgotten about.