The flaw with that idea is that multi-weight oil gets more viscous when it gets hotter. 10W-40 is 10 weight cold and 40 weight hot. Also, lifters pump up with oil pressure and oil pressure drops in the engine when it gets hotter.
Something else is changing. Most likely in the idle A/F mixture or possibly in the distributor timing. I'd suspect the A/F first. Especially since you added the PCV system and that changes air flow through the intake. It may also be changing the fuel load into the cylinders, especially if you are getting fuel wash-down past the rings that is then being inducted back into the intake.