You may not have installed one of the spark plug boots fully which could be causing a misfire concern.
The oil won't foul the plugs unless the engine is burning it real bad (worn rings, valve seals, etc.) - does your engine burn any oil?
The only leaks you could possibly get from the intake are coolant or vacuum. There are no oil passages in the intake.
Again, check for porosity along the I.D. of the spark plug holes (below where the spark plug boot would seal against). I'd be willing to bet oil is migrating from the high-pressure feed gallery through the porosity and into those holes. If the truck has gone 100K+ miles and they only have a minor amount of oil in there, I would just clean them out, install new plugs and forget about it.