The only time I've run across a stubborn P0174 code was on a M54 engine that had a DISA valve that was not sealing well. German auto solutions sells an O-ring for around five bucks for that purpose. It fits really well and that solved the problem I had. I know it doesn't seem related to your oil pan gasket service, but that's where the air leak was after ruling out the intake hoses which do develop cracks over time...

__________________
'06 X3