With all the filling up on oil I do at the gas station, I've misplaced the o-ring that goes between the valve cover & the oil filler cap -- any idea where I can find/buy a replacement? Is there any harm in it not being there?

 

Stop by the dealers or try home depot.

 

The oil cap seal is important for 2 reasons - 1) prevention of oil leakage from the valve cover, and 2) prevention of loss of the oil cap. The rubber "O-ring" seals the junction and helps to keep the cap from vibrating loose. I suggest getting the seal from the dealer because it isn't really an O-ring design, it is square in cross-section.