Yes stop leak stops up everything else too. Yes to changing the oil/filter.
Also check PCV. With engine idling the PCV valve should have strong suction with valve removed from valve cover and your finger over the end of the valve.
To check the inlet side of the PCV system...again with engine idling, pcv attached to one valve cover, and inlet breather on other valve cover...place a scrap of paper towel over inlet to breather. The PCV suction should hold the paper to the breather inlet, if the paper falls off, there is not enough "draw" to the system, maybe breather is plugged up, excessive piston ring blow by, etc.
In auto mechanic school we were told it takes about 15 miles of steady driving to remove the condensation that developes overnight, thats ONE night.