Gypsie wrote:My recollection is that it starts out round but will become squished and set into the shape of the void that it fills while it sits there heatin' and sealin'.
Nope, it starts out with a "D"-shaped cross section, with the flat part of the "D" on the inner circumference of the ring. Regular round-section o-rings don't seem to fit in this slot very well and can make insertion of the distributor a real challenge if the ring thickness is just a skosh too large.
They eventually assume a rectangular cross-section as Gypsie notes, and then they get hard and brittle.
I could have sworn that some vendor (AirHead Parts, perhaps) was selling a Viton version, but I can't seem to find it. As long as your old one is still pliable it should be okay to use.
This is one of the few parts which are common to both the type 1 and type IV engines, so don't worry about sourcing a type-IV-specific part.