Thanks for the info, everyone! My current o-rings are orange, so I guess they are silicone? 

I have a suspicion now as to why it was getting eroded.  When I took the head off, I noticed that when head had been tightened down and the O-ring was compressed, a small portion of the o-ring was squeezed to the edge of the groove and pinched between the head and cylinder surfaces.  So while the metal surfaces were very close, they weren't completely touching. I imagine that this allowed the o-ring to "see" the flame slightly.  I hadn't thought about the metal surfaces not fully touching as being an issue, as long as everything sealed up just fine (checked with leak-down test), and I expected the o-rings to be able to handle the combustion similarly to how a gasket would.  I guess I was wrong about that.

I'll have to order some new ones and make sure none of it is getting pinched between the head and cylinder surfaces outside of the groove. See if that solves it.

Thanks again for the help!