Unless you are using it for patch lube there is no issue with petroleum in BP arms and might not be then. So long as the fouling is removed before oiling the bore. I also stand MLs muzzle down after oiling to keep oil from puddling in the breech or running into the stock.
If you clean all the fouling out then there is no issue with petroleum. Used them for decades with no issue.
If you have a cupped breech, for example. and let fouling remain there, oil it the subsequent heat and pressure from later firing will cause problems. I fit continues to build up it will fill the cup and in some cases require debreeching to remove the stuff. If you use a commercial BP patch lube or solvent look up the MSDS on the WWW. They are generally 80-90% water and sometimes petroleum distillate. Which is what is in WD-40. Lubes the penetrate usually lake the ability to protect high pressure contact points from wear.