Assuming they are lacquered finish like from the 70's, I use polyurethane. I've found semi-gloss Minwax to work well but haven't tried any other brand. For earlier stocks I use Minwax Tung-Oil Finish.
+1 on the Minwax Tung Oil for early grips, for later vintage grips I use Deft Semi-gloss spray. Easy to apply and durable, rub out with grey Scotch-brite and paste wax buff with a micro fiber cloth.