First off you shouldn't have oil in the upper intake manifold at all unless you are getting a lot of oil through the pcv valves. Do you have a lot of blow by? have you checked compression? PCV valves new? The idle is controlled by the IAC valve on the back side of the throttle body. Someone could have turned in the screw on the throttle body too far causing the throttle valve not to be able to close far enough for it to idle down. If you've really tried everything else including the TDC check, then the next step is what I am planning to do.
I am going to pass along what JWaller suggested to me. Remove the throttle body and upper intake. Remove the IAC valve from the throttle body. I would probably go ahead and remove the lower intake too. CLEAN everything very thoroughly to remove all carbon deposits and see how oily the intake is inside. By this point, your probably considering the same thing I am .....L35 heads. but anyway. The idea is to make everything look new. match port the upper and lower intake. you may want to consider an EGR blockoff plate to keep deposits from biulding up in the future. John also suggested to just do away with the pcv valves and run a push in crankcase breather in the valve cover. I'm kinda hesitant to just do away with the pcv's though.... I would at the very least make sure you use the correct new pcv valves with new hose. I used hydraulic transmission hose. Basically the idea here is that any oil that enters the combustion chamber acts to lower the octain of the fuel, causing detonation. Reassemble with the best gaskets you can find. i would ask an experienced syty mechanic like jwaller what gaskets he reccomends.
Good luck
I am going to pass along what JWaller suggested to me. Remove the throttle body and upper intake. Remove the IAC valve from the throttle body. I would probably go ahead and remove the lower intake too. CLEAN everything very thoroughly to remove all carbon deposits and see how oily the intake is inside. By this point, your probably considering the same thing I am .....L35 heads. but anyway. The idea is to make everything look new. match port the upper and lower intake. you may want to consider an EGR blockoff plate to keep deposits from biulding up in the future. John also suggested to just do away with the pcv valves and run a push in crankcase breather in the valve cover. I'm kinda hesitant to just do away with the pcv's though.... I would at the very least make sure you use the correct new pcv valves with new hose. I used hydraulic transmission hose. Basically the idea here is that any oil that enters the combustion chamber acts to lower the octain of the fuel, causing detonation. Reassemble with the best gaskets you can find. i would ask an experienced syty mechanic like jwaller what gaskets he reccomends.
Good luck