undy, you beat me to that possible cause. If it is a leaky intake gasket it will show as oily traces in the cylinder port and none (or very slight traces) in the intake port, and carbon deposits and crud on the top (back side) of the intake valve.
One last suggestion: if you have Edelbrock heads (not sure if this happens on other brands), putting the rocker bolts in the "rong" hole will cause the bolt to "punch through" the top of one of an intake port in the head, and the motor can suck
oil past the threads; again, it will show as an oily trace running from the punched hole in the top of the port to the intake valve.