Thanks for all the discussion. I have to admit that the PETE bottles I use don't seem as thick as the HDPE bottles, so maybe the difference in performance isn't quite as great as the raw numbers would indicate. My wife gets bottled water in PETE bottles that are so thin it's like holding a water balloon.
Just as an aside, I also looked up Saran wrap since that has been mentioned as a way to improve the cap seal on bottles for storing photo chemicals. It turns out that Saran is an exceptional oxygen barrier which is why it keeps food so fresh. Other plastic wraps use less effective polymers. Unfortunately, Saran wrap is no longer made from Saran (polyvinylidene chloride) for environmental reasons. Now they make it from ordinary LDPE.