Re: O-rings on lower end fill and vent screws?
Your motor does not take O-rings stock, unless someone has changed the drain/vent plugs and I doubt that very much.
The correct gaskets for your application are either Sierra P/N 18-2945, or Mercury P/N 12-19183-3, at around $.70 each for the Merc part.
Your local Napa should be able to get the Sierra part for you.
HTH..........ed