I would not add any lube to the wads in BP cartridges. Lube added in an amount that might affect the fouling might also contaminate or kill some powder. Most of us who shoot BP cartridges carry a spray bottle of moose milk or plain water. Spray this down the bores after every few shots. It will keep the fouling softer and some of it will shoot out, as you go. Moose milk is just water with soluble oil added in the ratio of about 1 part oil to 7 or 8 parts water. The oil will help prevent rust from forming, during the day. Shooting a round or two of skeet, after the black, using low pressure smokeless will clean out most of the fouling.
The above advice is assuming you're shooting skeet or trap. For just hunting, where you might shoot a half-dozen shots all day, I
wouldn't bother with carrying the spray bottle. I've been happy with 60 grs. GOEX 3f and 7/8 oz. shot, 68-70 grs. 3f and 1 oz. I haven't used 1-1/8 oz. for years, but with it, I'd use 75-80 grs. 3f.
