Is there any oil burning? If so, then it could be a piston ring or seal that's dead. Have you noticed any leaks, fluid under the car after you leave the car over night? If so, it could be your oil pressure is too low. If there is too much air getting in to the cylinder that could also be a problem. If he didn't put those bolts back in I would do that and if there is still too much air, try replacing the gasket on the intake manifold.