I'm wondering why you don't try it out. Simply take two pieces of steel, (not galvinazed) coat with what ever leave it out side for 3 days and then check it out. I would bet the sythetic motor oils will be right up there with your EXPENSIVE gun oils.
Bill
Check the links. Synthetic motor oils are poor at rust protection.
I did my own tests years ago. Used all types of oils and greases. RIG was good, and even WD-40 did a reasonable job. Motor oil, not so much.
From the 6mmBR test, 15hrs
Top row, left to right. Control Piece, Break-Free, Strike-Hold
Bottom row, left to right, Rem-Oil, Slip2000,
Mobil 1 (15W50)
19hrs salt spray
Top Row: Left to right. Control, FP10, Corrosion-X
Bottom Row: Left to right. Eezox, Breakfree.
Top Row: Left to right. Control, FP10, Corrosion-X
Bottom Row: Left to right. Eezox, Breakfree.