Well, what is the specified Idle for your car? Your idle speed should remain the same regardless of the induction type. sometimes, idles tend to drop down when they are running rich.. If you have Mediocre fuel managment, you could be loading up at idle and have a lumply/lower than normal idle speed. i see it all the tiem on DSM"s w/ a vented BOV... ive had it happen to me, so i know the symptoms.

how lean are you at idle?

a good rule of thumb is 10 psi of oil pressure for every 1000 rpm, i think if you are running at least the 10psi @ idle, it should be ok... the thicker oil is gonna increase your pressure. you probalby shouldnt deviate too far from the OEM's recommended oil weight. So if it calls for 5w-30, you should probably use that. the fact that you have more oiling orifices would account for a slightly lower oil pressure given the same weight oil as before.