I don't have trouble with the swingarm oil leaking, but my experience is with bikes I've built. Here are some things I do that I think cause them not to leak.
1) Cradles are always blasted clean and never powder coated.
2) The area where the o-Rings or later-style seals will touch the cradle are perfectly smooth or the cradle is not used.
3) Before masking for paint the rear edge of the cradle where the o-Rings or new-style seals will be pushed into place are rounded over and made very smooth (beveled if you like).
4) When masking, I want as little bare metal as possible showing once the swingarm is in place. This prevents rust from creeping under the seal. I mask the area, put in a spindle and original style seal plate and cut around it.
5) The oil nipples supplied for a while were really grease zerks and the threaded portion was WAY too long. The caused the timing side end cap to not be able to sit properly on its o-Ring and it ensured a leak. AN has fixed their part of this - no idea about others. The too long fitting can be used but it requires 3 washers to space it out properly.
6) When putting the bushings and dust covers (original setup) I use a thin smear of RTV between the bushing and cover to prevent weeping at that point and I use new covers if the old ones are no flat and perfect. Also, if there is paint/powder coat on the edge where the cover or new-style seal will sit, it can cause weeping there and it needs to at least be made smooth if not complete removed.
7) If the swingarm is powder coated or too much paint is used, or there is paint/powder coat on the edge that the O-rings will not sit in their groove properly and therefore the endcaps will not be properly sealed causing leaks.
8) If the spindle bore in the cradle is wallowed out and the swingarm can twist in the cradle, it's only a matter of time before the o-Rings or new-style seals will be worn and leak. I'm not a fan of Kegler clamps "just because", but in the absence of a good cradle, I install them using parts from McMaster-Carr.
In other words, lots of root causes of leaks, all I know of are preventable.
At this point, I have no solid opinion on the O-rings verses new-style seal. The O-ring version seems more robust to me, but also more likely to leak if either the cradle or swing arm dimensions are off.
An experiment that coming is x-Rings. Most (all?) O-rings sizes also come in x-rings. For instance, the 6 O-rings (06-1900) specified for the fork ears are easier to work with in the x-ring version.
