My opinion:
Petroleum based oils and solvents do not mix well with black powder combustion products.
I do not knowingly use any petroleum based oils or solvents on any of my black powder guns. Anyplace. Common sense says that using such products in areas that are not exposed to black powder combustion products (like anyplace outside a rifle bore) is not a problem, so my attitude is a bit obsessive. But that way I don't have to deal with making mistakes or the oils creeping somewhere I don't want because I used too much.
There are a number of rust preventative products made for use with black powder guns; Hoppe's No. 9 Plus for Black Powder comes to mind as a cleaning solvent. Many people use Ballistol. Thomson Center markets products called No. 13 and TC-17. Some people use vegetable base food oils (olive oil, peanut oil); they are not generally good for long term storage because they can spoil and become rancid, but they are superb for short term use. I use Bore Butter, but many people claim it does not work for long term rust prevention. So far I've seen no problems, but maybe someday...
As I said above, it's good to avoid exposing black powder combustion products to petroleum base oils. However, for long term storage it's perfectly ok to (thoroughly!) clean out the combustion products with hot water or your favorite solvent and then immediately apply a thin coat of oil. After storage it's important to clean the oil out before shooting the gun, but there's no reason that can't be done.