When I said oil was oil, I only meant it all starts from the same place. It's after that that matters.
Only new oil. If you're going to use pre used oil, you might as well throw in some sand while you're at it.
Heavy weights will probably cause problems, at least with semiautomatic guns where part need to move quickly. Too much would be a problem as well, but then again, too much gun oil would be a problem.
Gun oil is usually a very light oil, and we'll filtered. Sewing machine oil is even lighter, and as well or better filtered.
3 in 1 oil is a lousy gun lubricant, you can watch it gum up.
Wd 40 is lousy as well, and will eat bluing over time.
I've thought about atf Trans fluid, seems it might work well. Trans fluid is loaded with detergents, you don't see a dirty transmission inside. Also very light, and probably well filtered due to running through a valve body. Only problem is transmissions don't run near the Temps as inside an engine, so I don't know how well it would hold up to hot semiautomatic.
I had a can somewhere of usgi cold weather arms lubricant. Very thin, almost like water. Looked kind of like Trans fluid mixed with something to thin it. Smelled strange. I need to find that can again.