re: "...Wouldn't you expect to find oil leaking out (or a hissing noise) on the exhaust stroke?..."

On exhaust stroke, perhaps the hissing sound, but not oil leaking out unless the cylinder is filled up with oil (the plugs are located high in the combustion chamber, so how would a quantity of oil reach it?).

Regarding the sound, sometimes a leak is hard to hear -- I've got a nifty little sound detector (from IPD) for diagnosing/locating sources of suspicious sounds -- maybe someone you know has one, too, that you can borrow?

Anyway, remember that all of the above is speculation -- just trying to suggest ideas to help.

Good luck.