I was wondering which Cafelat Silicon Group Seal Gasket (size???) be the correct one for a Rancilio Silva? I see there are 2 options (red 8mm or blue 8.5mm) whether or not they would fit is another question I would like answered. I am not having any trouble with it, I have had the Rancilio for a few years now and was wondering if they should be changed on a regular basis?