MC18 is a damper rod not a cartridge fork so you can drain almost 100% of oil from it, however there will be lot's of black dirt still sitting at the bottom of the forks, even with forks completely dismantled it takes some effort to get it out.

I think 10w with 120mm will work for this (measuring oil or actually air gap from the top of the oil to the top of the fork tube, spring out fork collapsed)

what you will really benefit from is a RaceTech cartridge emulator, makes much difference and while you there I would replace the springs, bushings, seals... probably about $400 in parts by the time you are done but it will be like new. Just changing oil will not make any noticeable difference unless you make changed to the oil level.