the synthetic has been excellent at rust prevention. I'm no gunsmith but have been told specifically by one to only use the Mobile One synthetic. Not sure why but it sure works well.
Of course this free advice is worth exactly what you paid for it, but I'm happy with the results.
Even my general purpose oil can has had Mobil One in it for 30 years--since I came back from a tour in the Arctic. At 50 below, regular oil turns to taffy.