I bought the new O-ring (shaft seal) last May. It's still sitting in a box.....waiting. A service tech told me the rubber material had been changed from original (improved). I said "Great, I'll take one". That's all I know. Anything else would be speculation.
It's something I've seen many times with oil related items (worked with lots of valves and instruments immersed in oil). I have occassionaly had to switch compounds to gain better material performance. It's not perfectly predictable.
JR, your thought is valid for testing, but not really necessary. It will begin leaking again....I'm quite sure, based on my experience. I'll stay with the conventional oil for a good long while. I may be operating on borrowed time, as it is. I' m rather interested to see how long it will go before leaking again (if at all). Once it does, I will have to replace the seal.
FYI....if you have some leakage around your valve cover (which is common) try switching your oil type, but also make sure your cover bolts are properly torqued. I had a little oil seepage there around 10,000 miles, but discovered the bolts were not tight.