In my experience, installing the friction plates dry works best. The steel plates will adhere to the friction plates well when their surface is roughed up a little bit also, either by bead blasting, or scuffing on a rough surface to cut some light scratches in the steel driven plates.This will enhance adhesion, which means you won't have to pre-load the clutch springs quite as much, in turn leading to a lighter pull at the clutch lever. I also recommend using plain 20w50 oil in the primary chaincase. Be sure not to saturate the friction plates with too much oil, as they will be prone to sticking together after periods of inactivity. The oil is in the primary chaincase to lubricate the chain, and to some degree, helps in cooling the stator. I worked at a Harley dealership for a couple of years, and we always soaked the clutch plates in oil for an hour before installing, but I don't recommend it with the vintage Brit bike clutches.
-Dave