I find a significant amount of oil under the car particularly after extended driving at highway speed (not so much after short local trips). After initially "repairing" the turbo and replacing the O rings, the mechanic tells me the car needs a valve cover gasket which he says is a pretty big job. To prove this he has shown me oil pooling on top of the engine under the black plug cover which he says then drips down the back right (driver's) side of the engine. I see no oil on the sides of the engine near the gasket only on top. Does his explanation of "valve cover gasket" sound right?