Keep in mind fellas, that Mobil 1, while being a very high quality oil, is no longer a true "synthetic". It is refined from regular dino oil nowadays. They switched after losing a suit to some other manufacturer over the use of the "synthetic" label for non-syn oil.
I use Mobil 1 10W-30 myself, in my Mustang. Works well.
For firearm use, I doubt you need to go crazy and use Amsoil, Royal Purple, Mobil 1, or any of the fancy oils. Regular motor oil is a fantastic lubricant, and is designed to be used and abused in very tight clearances in engines (0.002-0.003" at the tightest spots, usually). Lubricating firearms is about as simple a job as you can give it!