If the viton seal works for you then I highly suggest you keep using them.
I know what works for the vehicles I work on and plan to keep doing what I know works well.
But saying E-10 is a good fuel is talking with blinders on.
Fuel economy goes down, rubber parts are affected whether or not any one else seems to think so.
As for sealing the tanks with the gas cap, most of these old rigs use a vented gas cap. Not usually a problem as long as the vehicle is run regularly.
But if you've not had problems with E-10 fuel, you indeed have been quite fortunate.
