I have an assortment of the green "high pressure" O-rings for car/truck HVAC use. They appear to be viton - rather higher durometer than the usual nitrile - so would be appropriate for Guzzi crank sensors and that kind of stuff. 390º was their designated limit for that material.
Boeing experience you say? I was in tooling in the 70s until I found a steady job in the government Dad spent 38 years there, ending up in Experimental and Test (B-3410 for any Boeing types) in a small building adjacent to the runway. He enjoyed the heck out of it, as the shop was a microcosm of the manufacturing process, with early CNC, heat treat furnace, autoclave, A&P welders etc. He told me stories of the various times he had to demonstrate to certain engineers that two objects cannot occupy the same physical space.