I dont know about RS turbo's, but if its popped the oil cap off then there is a shitload of pressure there shouldnt be in the bottom end.

Either you've got severe ring/piston damage - or the rings are on their way, and the crankcase breathers blocked.

Whatever you do dont run it, cause you'll chuck metal filings and fragments up round the head & probbably turbo too and completely f*ck it. I'd be inclined to get a compression tester on it, they're about £20 for the cheapo ones, see if it shows you're massively down on one cylinder. Then depending on how bad pull the head and have a look see.