If you're wanting to condition the frog, I know Corona ointment advertises on the bottle that it can be used for hooves. I've never tried it personally, but had good results for it wound-care wise. I'd avoid oils, the hooves might look wonderful but I doubt it would really do anything other than make them look fancy!
I have used hoof conditioner, and I definitely think it helped her frog stay conditioned...so, I wouldn't say that everything you put on the outside is useless. But (as others have stated), for healthy hooves, keeping them trimmed and feeding well is very important as well. :)