Muzatko - Seems a lot of guys in here use some type of preventative oil. I'm not sure how these petroleum based products react when combined with residue from Pyrodex or Triple 7 powders. My experience with black powder has shown that these lubricants when mixed with black powder fouling leave a tarry like mixture in the barrel that is tough to clean.
I personally have been shooting BP for a long time (since the early 70s) and I use T/Cs bore butter in all my MLs. A couple of my MLs I've had for over 25 years and they show no signs of rust. Of course, every time I shoot them, they get a good hot soapy water bath and hot water rinse, dried, and treated with the bore butter.
Some of the guys in here are very knowledgeable about Muzzleloading and I have taken their advice many times. But I just can't seem to get myself to change from the bore butter. I live by the adage "If it ain't broke, don't fix it." I know "An ounce of prevention is worth a pound of cure." would probably be the response to that but until someone shows me that Pyrodex and Triple 7 fouling does not gunk up when mixed with a petroleum based lubricant/rust preventative, I am not going to use one.
Maybe I will do my own research on this and let you know what I come up with. I know I'm an old fart and set in my ways.