back pressure causes 300ZX turbos to leak but it is a common source of discussion. Generally the advice is to replace the cats if the smoke is not too bad and it will often fix it. If you replace the turbo's buy new for best chance of not having a repeat of the problem. Be sure you do the pressure test too. It may be a bad sending unit or gauge but there could be another issue there with the high readings. I once had an engine builder tell me that excessive oil pressure could "wash" the babbit off the bearing shells.
