I actually contacted Albert Von Schweikert about what I should use on the surrounds of my Von Schweikert VR-5 HSE speaker surrounds, and this is what he wrote back:
Hello Juan,
All of the VR-5 HSE speaker components use Kurt Mueller rubber surrounds that have a twenty year warranty. The Germans know how to make good parts! The Audax midrange and SEAS woofers use these special rubber compounds for a very long life, estimated to be more than 30 years. If you treat all of the butyl rubber edges, they will not crack or dry out for an estimated 50 years!
Buy the liquid made by CAIG from Parts Express; we use it to protect the foam rubber surrounds of the VR-5SE and VR-5 Anniversary models, as well as the VR-9SE Mk2 and VR-11SE Mk2. It works very well!
Cheers,
Albert Von Schweikert
President, Chief Design Engineer