GTI RPM and Turbo

My boyfriends fuel line has been getting oil leaked into it and then that messes with the turbo, tonight he was going 30 in a 50 and pushing 35 RPMs. He's taken it to multiple mechanics and it's never gotten fixed.
Problem with the turbo: when he goes to accelerate up a hill or onto the highway the turbo gets hit and then it backfires and smoke comes out the exhaust, if anyone can help I'd appreciate it a lot.