I put a small powder charge (about equal to 25gns FFG) into my new (to me) muzzleloader after I cleaned it from squirrel hunting yesterday. I topped it with a patched round ball.
The bore had been run through with Fluid Film and the patch treated with it. I've been using it to treat patches as the petroleum thing doesn't mean much to me - I use petroleum jelly as an ingredient to a bullet lube I developed for my cast .452" bullets, before I went to my proprietary "J.S. Lube".
It's waiting to go out squirrel hunting again - and as I need to find a piece of wood for my powder horn I'm making, it might as well go with. When it does, I'll fire it off and see if that little bit effected it. I'm betting not.
I'll report back.