Depending on the year of airhead, though I think all R65s are the same in this regard, the metal tube that the pushrod goes through has a brazed on circular end that pushes against the rubber seals. Sometimes this donut of metal starts to come loose and the seals loose their pressure against the crankcase. Some people have found that if they start putting synthetic oils in an engine that has always used dino-oil that seals start to leak, and reverting back to dino oil eventually cures the problem. I don't want this to turn into another dreaded oil thread, so I'll stop discussing that right now.
It isn't a HUGE job, but as you guessed, you have to at least pull the heads and slide the cylinders so you can get at the pushrods and tubes, seals, etc. so it is a bit time consuming. Just go slowly and methodically so that you don't end up with a leak afterwards - which means do it all over again!
Personally, I wouldn't touch the rings unless you are having some oil consumption/smoking issues. It probably isn't a bad idea to replace the o rings at the base of the cylinders at the same time.
Others will offer their input soon,
Mike
