I have a 1997 Nissan Sentra, I've just had engine work done: rings, head etc.. While driving along the other day I noticed a puff of grey smoke which was intermittent. It would smoke for a moment then stop for quite some time, then start again.. etc.. I changed the PC valve and still getting the same thing. I unplugged the hose from the PC valve and the problem stopped. My question is: Does oil viscosity have anything to do with the operation of a PC valve? I'm using 20w50 for the first 1000k break in. I'm changing the oil to 40w after.

Your car likely requires 5W-30 engine oil, which is what I would recommend. Higher viscosity engine oil will increase friction (heat), decrease gas mileage, and offer less protection. Various oils have different NOACK Volatility ratings, which can affect sludge build-up in the PCV Valve and hoses.