I've got what looks like a bit of surface rust appearing on the outer barrel of my 1911. Right at the end where it shows when the slide is back. Not sure if its rust or maybe because I got some Hoppes No 9 solvent on it.
Got similar effect on the outer chamber also.
Ant tips on how to remove this before I get to work trying 000 wire wool.
OK, so, rethinking things...
Maybe it isn't rust at all.
Maybe it's a kind of varnish, caused by—as you thought—baked-on gun oil.
In that case, it'll be pretty difficult to remove.
But if that's what it is, it isn't harmful.
If it is a varnish-like substance, try scraping it off with a very sharp chisel, using the sharp edge as if you were trying to lift a layer of paper up while pushing the chisel forward.
The shallowest angle possible would be best, because it will scratch the least.
Then finish, if you've caused scratching, with crocus cloth.