Until Royal Purple introduced a specific Gun Oil, I used RG21 for all lubrication except AR and Benelli bolt carriers.
They do the job and are the only pure synthetic oil I can find. Mobil One is not pure synthetic.
Anybody else use Royal Purple?
I use Militec-1 or Slip2000 gun lube (and very rarely CLP) on my guns and either Amsoil 0w-30 or Mobil 1 0w-30 or 5w-30 in my vehicles.
Royal Purple Race Grade 21 is said to be 5W25 weight.
I run Mobil 1 5W30 in my truck. Royal Purple is WAAAY too expensive for use in the daily driver. About $17 per quart but a quart lasts a long time as gun lube. The new RP Gun Oil is about $6 for a four ounce aerosol can.
Wow, wasn’t aware it was that expensive. I had to take a look, and Jegs has the regular for $6.99 and the racing is $11.99. I use slip2000/fp10 normally, but have used pedro’s road rage (bike stuff) and boeshield. Just mixed up a quart of synth/atf to work through as well.
I think you just convinced me to change vendors for RP oil! 
Happy to help, friends don’t let friends buy $20/qt oil