When the BJ/GSP fight happened, it wasn't illegal to grease your body. The rule was written something like "A fighter should not have EXCESSIVE grease on his face or body". It didn't say you couldn't have ANY grease on your body. I think it was generally understood but technically GSP didn't do anything against the rules.
Now - if a fighter greases his body, it would be illegal.
Oil bath prior to fight, still OK.