[Rev]

It's possible to get the gaskets in such a position that they don't seal at the bottom area of the intake ports. This could cause a vacuum leak sucking air from the valley area but I'm not sure if that would leak gas into the oil? Still, I suspect some kind of error in the reinstallation of that intake manifold. Take it off and look for some kind of evidence of a leak. You only use the RTV around those water jacket passages, not on the the intake port areas of the gasket.

My bet is when you remove the intake manifold you will see the problem. Let us know how it goes. If it's any consolation, I've had similar problems myself in the past and it was just a simple screw up on my part when setting that intake down onto the heads and misalligning the gasket in the process. Let us know how it goes.

Rev