Are they natural rubber or polyurethane? The modern Seiko straps (e.g. Z-22) are made from polyurethane. If the GL-831s are made from natural rubber, petroleum-based products (such as Vaseline) will have a deleterious effect. I suspect, but don't know, that polyurethane is more resistant to petroleum.I have put a thin layer of pure vaseline onto a couple of my GL-831 straps, optically they look much better than before but I am not really sure if this is good for the rubber though!
I won't take the risk of boiling them because I am afraid they may dissolved as they are quite old...