At the warehouse, they mentioned that this was a problem sometimes reported with this design (and the reason for switching to plastic D-rings on the new model), but that since the normal Ristretto strap has a plastic clip, it should be fine.
Well, it turns out that the plastic clip is a plastic coating on a metal clip! And it wears through quickly and then squeaks!
I've tried the pencil lead trick, but it doesn't seem to last long. Are there any good alternatives? I have some machine oil, but I'm worried that it might stain the bag. Maybe plastic carabiners to connect the strap and hinge, but would they be strong enough? I think I'm going to upgrade to the Awesome strap, but I don't think that even has the figleaf of a plastic coating.