The Usher diagram above says to change the viton o-ring annually or when needed. Caution - the reference number for that o-ring is 2-325 on the upper diagram, and 2-235 on the lower diagram. I think -325 is the size you want.
RV6/2001 built 2000/sold 2005
RV8 Fastback/2008 built/sold 2015
RV6/August 2022 build - Flying