If you are just draining and refilling the forks, 190 ml of oil is required per fork leg, if you disassembled the forks, for a seal change, bump it up to 220 ml .
The standard fork oil for the R65, is 7.5 wt, only available from a BMW dealer, or repair shop, haven't seen it anywhere else .
If you can't source it locally, 10 wt oil will work as well .