As a follow-up, the white, very soft, Viton, o-ring seals very well, and runs very smoothly when you are using it, but, it is so compliant that it tends to push the oil out of the interface between the plastic and the o-ring, and it then wants to stick like a rubber tire. You can always pop it loose, but the first action of the day and it is usually stuck.
I reverted to the medium durometer silicone, and that doesn't have quite the same issue. The harder Viton is also fine. All are completely impervious to model fuel, as far as I can tell.
I now have a very large number of acceptable Carl Shoup o-rings that can be used as replacements, that is, 3/32" cross-section -125 sized rings. If someone wants them I will have them with me at contests, or, send me a SASE and I will send you a couple for free, until I run out.
Brett