Reading the BP posts here, I've seen them say you shouldn't use petroleum-based oils and standard gun-cleaning solvents with black powder. Smokeless wants the gun oils and solvents that are common now; BP wants "natural" oils for lubrication, tallow for grease, soap and water for cleaning.
I haven't seen anything to say that it's a safety issue, though, or that it will damage the gun. From what I've read it's just a matter of a mess. Might be a little worse to clean up.
Myself, I used "regular" gun oil with black powder guns, not knowing there was supposed to be any difference. I didn't have any problems with it. Perhaps if I had been shooting hundreds of loads all day it might have made a difference.