Well done, all ! Very helpful information.
I'm feeling confident enough to add the oil cooler o-rings to this thread.
Parker series 5-204 o-rings are replacements for the Duc o-rings leading to the oil cooler on some bikes. It seems that they changed the fittings over the years. Mine had two o-rings on each hose fitting that mates to the threaded fittings just forward of the oil pump.
These o-rings are NOT the standard AS568A type. Those have a minimum width of 1/8in. The Parker series 5-204 are 0.036in wide. You can see these in the linked spec:
The ones I orders are non-viton and a tad smaller I.D. compared to the originals. The Parker series 5-209 might work and be the same I.D., but I have not tried these.
It seems that prices can vary wildly for these. I found 12 of them for $20, delivered.
Have a good one.