Motor oil shouldn't be used on firearms as it'll turn to a solid form like a varnish when left on. Fine oil will to but it takes longer. The CF didn't issue motor oil for cleaning anyway, we issued a specific rifle oil made by Esso which was a oil like sewing machine, very fine... Some of the older more ignorant ones would use gasoline and motor oil mixed 50/50 for cleaning but that practice was moved away from in the mid '70s with the FNs. We issued specific products for weapon preservation. Matter of fact we even taught NOT to use motor oil.
Don't use motor oil...