Another possibility for your solution might be a mix of Ballistol and water. It is one of the only oils that I know which mix well with water and should work as a cleaner and rust delay (note I did not say preventative). I just got a gallon of Ballistol and am somewhat impressed with my first uses of the product. I am considering it as a long term storage oil and since the traditional guys use it as a patch lube it should not cause any mess as described using petroleum based oils (although I am not so sure that really happens). I think the petroleum based oils used with black powder might just not be as bad as the old wives tales say. Any strong rust preventative might just attack bluing since it is just controled rust.