Hiho and thanks for your replies!
I don't think it's a matter of luck getting rid of oil marks that way.
All the proposals may work as well (O-Ring, rubber coated washer), but if the original seal doesn't work what it is for, you should not blame that ring seal.
As Oldfeller pointed out, the reason of leaking and destroying the lip of the seal may be roughness, corrosion or radial play, he called it oscillation. Which might be as well as de-centered action, maybe from unexact make.
There are many S40 or Savage's that do not leak, even if there's nothing like an O-Ring or the "Simmerring" as it is called here in Germany. My Engine in the sidecar-Savage does not and my "Solo" doesn't, too. Just like others I know of and also know, they have no seal mounted.
Others wonder and complain about remarkable amounts of oil leaking down from the cylinder head cover. Panic!! Mostly then the oil crawled up in the speedo cable and does not know where to go then with reason.
If the speedo is oil stained from the underside and one remarks oil reflow flom the speedo cable it's easy to explain.
This little a**hole
)) goes away and often got lost every second attempt to take off the fuel tank.
Maybe it's a difference in the clearing of the speedo-shaft in the engine case if it will leak or not without sealing and maybe there are differences how the oil gets out of the gear shaft bearings. The lubrication of the speedo drive shaft is more random from oil drops flying around in the case and not fed from the oil gallery.
Hope you get me right, I'm not a native speaker of the english language. So I beg your pardon!
Regards, Lou