O-rings dont allow enough thread engagement and dont allow you to tighten the "right" amount. (eventually the O-ring joint will loosen and leak). You get the "right feel" with the paper gasket. They're cheap.... use a new one each time.
First time changing sterndrive oil, West Marine guy said "Merc?" I said, yep. He gave me o rings. But when I removed the screws, there were hard (fiber) washers, so I cleaned them up and reused them.
Part numbers: 12-19183-3 or 18-2945-9.... Did you know these get torqued to 30-50 in-lbs? (me? I'd ask the West Marine "guy" to show me the part number for the o-ring on your drive's drawing.) (note...my manual does in fact refer to "sealing washer or O-ring" in a couple of obscure places, but I have never seen an O-ring used on the parts sites I shop at).
Don't ever take advice from anyone at West Marine. When I needed some the guys at Napa took them from a package that had the plug in it because they didn't have them in stock at that store. They still only charged me for the gasket and replaced them when they got them in I guess.
Some drives have used both in the past. The O'rings are an option but not a good option due to in the past, it has been found that the "vacuum" (if you will) of the prop could cause the Oil to be sucked out past the O'ring while on plane.
Two boat owners I go to for advice or second opinions both have MerCruisers and they both use o rings, so I was taken by surprise when I found washers. I almost replaced them with o rings, but since the oil looked good, no water, just brown from age, I stuck with the fiber washers. Thanks for the feedback!
Trying getting an oil filter, water separating fuel filter, spark plugs, anode set, marine battery, 2/0 battery cable, 25 feet of 3/4 anchor chain, 100 foot docklines, block drain plugs, etc. etc. etc. from a Merc. dealer on Sunday around noon. Probably won't happen yet West Marine may have exactly what you need instead of waiting.
Maybe even would have had what you needed on Sunday to go boating on the holiday today.
Price isn't everything and in some cases being able to get what you need when you need it is worth almost any price.
I've seen and installed plenty of the plastic washers, but NEVER an O-ring on a Merc.
I honestly don't remember whether it was OMC or Volvo that used an O-ring but the plug has a ring land cut into the underside of the head specifically for an O-ring.
Any plug that has a flat bottom head must use either the paper or plastic sealing washer, definitely NOT an O-ring.
The lasest issuance from Merc are like a rubberized flat washer. Been using them for about 10 years with no problems. Bit more expensive, but re-usable. They are a yello colour, with a small sealing ridge....
The washers I found were a blue green color and we're not plastic or rubber, more like fiber or dense, coated paper. I cleaned them up and reused them cause all I had were rubber o rings that the West Marine expert sold me.
