Very normal - your pcv valve (positive crankcase ventilation) pulls oil-laden vapors from the crankcase into the intake whenever you have vacuum in the manifold which is most of the time for street driving. There is a baffle that's supposed to go on the bottom of the lower to limit oil from getting to the pcv valve, and also an oil trap (cylindrical screen device) that sits under the grommet that's designed to knock the oil out of the air so not as much reaches the intake. But some is bound to get in. If your baffle is in place, and your pcv valve and the screen underneath are all clean or new and doing their job, there's nothing to worry about. If you didn't renew/clean the pcv components when you swapped the manifold - it would've been an easy time to do it. A new grommet, valve and screen will cost you about $16 at your local Ford dealer.
