I agree with Frenchtown. The manifolds need surfaced before installation.
The oil on the intake side is from the poor sealing on the valve guides GM used.
A cheap fix is to install umbrella seals on the valve stems. This will require spring/ keeper disassembly while on the engine. Basically, apply air pressure to the cylinder with the valves closed and then compress the springs and remove the keepers allowing you to install the seals.
The best repair is to machine the head for positive seals. I use Viton seals. But this is removing the head for machine work.
