I got a used Trinity Tru Lathe to play with. It came with several black O-rings for driving the armature. Two of the O-rings are quite stiff, not sure because they got old or that's just how they've been since new. No wear mark on them so maybe because they were too hard from the start to be useable. The one O-ring that is soft has wear so I think that's the right one to use. They are all about 1/8" thick O-rings. Looking up similar O-rings, 70 durometer seems to be very common, are they suitable?

yes stiff O rings are no good. local hardware store should have a usable O ring not sure the 1st you listed will work..