My mate has bought a Vectra C 2.2 53 Reg and he said he has an oil leak from the back of the engine thats dripping onto the exhaust. He said when the car is parked on the drive he can't see any oil patches at all. It's only leaking when he driving and he can see oil burning from in the car when he is in it. What can cause the oil to leak onto the exhaust? Thanks