Croupier

Hi everybody, I'm new here but as I glanced (and searched) around I could tell you've all contributed to a valuable resource. My bikes a 1975 XS650 B but I know for a fact that it has the later model forks that are beefier (1978, I believe). I'm basically slowly checking the maintenance on everything since I bought the bike last summer and need to ride to escape these insane gas prices soon. My question is pretty simple: the local Yamaha shop has various fork oils and fluids (including an awesome looking Japanese import, OEM, Yamaha, tin canister of fork oil for $30) and I just grabbed some 10wt Rock Oil figuring it was probably going to work but I really didn't even notice that there were various viscosities. Which viscosity is best for my set-up? Does it depend at all on preference, because I really don't have one. Any suggestions or tips would help (because the 650Wiki and my Clymer manual didn't and, to be honest, the kids they hire at the Yamaha shop are... dumb... sorry).