wrote:
If oil had gotten onto the inner tube, I'd expect it to feel
softer and to look shiny and darker than the surrounding dry
rubber--the rubber soaks up oil and loses tensile strength.

Cut two chunks of inner tube, dunk one in oil for a few
days, and you'll see what I mean--the oil-soaked tube feels
and looks quite different.

Good point. I wasn't thinking about how an oiled tube would be different.
There is an irregular shaped bump in the rubber surrounding the valve that
got me thinking that something might have happened to the rubber, but
looking at another (good) tube now, I see something similar. But the good
tube has much better support around the base of the valve. I think this
might be one of those defective tubes that Jobst talked about. It has just
been handled gingerly enough that it lasted so long.

I'd be pleased to see signs of oil damaging an inner tube
(well, sorry for you, but pleased in a ghoulish way), but to
me your tube looks dry and untouched by oil.

Yes, darn for you. I wish that I could say that I was able to tear the tube
up into bits with my bare fingers at the valve...

Because it's unlikely that the valve-hole area just happened
to be right under the leaky shock all the time, I wonder if
the pictures actually show a non-oil related problem. The
dark smears on the inside might be where the thickened
rubber bulge of valve-area acted like a motorcycle-rim
security-bolt. That is, the smears may show where the
stiffened rubber patch around the valve-stem was causing a
little trouble.

Good call, I think. The patches are shaped like the stiffened rubber patch
of the the tube.

So, maybe this says something about the difficulty of actually getting oil
on a tube and weakening it enough to cause problems? There was quite a bit
of oily residue on this rim, more than I imagine would be left on a rim from
oiling nipple seats.