I was talking with my brother the other day. He's been homebrewing much, much longer than me. We got to the topic of HDPE fermenters, and I mentioned that HDPE is much more oxygen-permeable than PET or stainless steel (he's been fermenting in his Speidel for years), and that I recently started fermenting in cornies.
I told him I was worried about oxygen ingress in HDPE fermenters, especially with lagers, and not so much with ales. He said as long as there's positive pressure (i.e. outward airlock activity), there shouldn't be any oxygen coming in through the walls of the fermenter.
It made sense to me, but I have no idea what the actual science is on this. Any ideas?