I'd let em sit for an hour or so, so that it soaks in the plate material and is prelubed so to speak going in there and won't rob oil away from the motor. Just use whatever oil you run in the bike.
i take a gallon from milk cut it in half fill half way with new oil that you will use in the bike after words put the new clutch plates in it and start tearing down the old clutch. if your fast i would let soak for 15-30 minutes even if you get the plates that you dont have to soak in oil there going to be in oil anyway you my as well soak em