Interior and exterior.. why syn. engine oil, cause I got gallons of it
a good conventional would probably work as well. Wood, I just use lemon oil or something similar, I don't much care for slick wood.
Why......well from a 25+ year truck mechanics point of view...
Find me a worse environment than inside an internal combustion engine for lubrication. I deal with and build engines that last over a million miles between rebuilds. Engine oil can survive high oil temps, temp changes, a turbo spinning 100k rpm and turbo turbine temps over 1200 degrees, sucking in hundreds of gallons of water from the atmosphere, the corrosive effects of combustion and end products and make a $30k engine last that long, why not a $300 rifle.
Besides and no one will do it, fill one of these engines with Remoil, FrogLube or space gun oil of your choice, and let's see the engine hit the million miles mark, heck let's just see a typical single coast to coast freight drag maxed out at 80,000 pounds, or go heavy hauler, 120,000 lbs and just do a simple lap through the Rocky Mountains..
Besides...what did commies use in the field?? Bet normal engine oil was really easy to find and get and it worked, if 40-60's grades of engine oil worked this well, look how much todays oil has evolved. 3000 mile oil changes, that was the norm 20-40 years ago....
I do 10,000 mile oil changes on my Honda and it's beating on the 300,000 mile mark. And if you look at it, what's a gun.. in the basic broken down principal, it's a very crude engine. It uses a fuel to produce power and do work. Guns and engines both produce abrasive carbon... both can create acid, guns from primers, engines from burning hydrocarbon fuel and high combustion pressures and temperatures, some acids an engine produced in operation will eat metal, like main and rod bearings etc, everyones favourite smog stuff.....nitric acid and sulphuric acid. Changing engine oil and cleaning a gun both remove the crap produced during use.