Cafelat and LF's espresso gasket for E61 groups is made from non-toxic food grade FDA silicone. This makes it better than rubber gaskets since silicone doesn't have any smell or taste and is easy to change in and out.
Cafelat spent months testing the espresso maker gasket so it can retain the strength of a rubber gasket but can also create a good seal effortlessly.
When replacing gasket remember the flat side lays face down.
Both the 8mm and 8.5mm will work on the E61 group machines. It is your own preference as to which thickness you prefer. The thicker the gasket the tighter the seal with the portafilter. When installing a new group gasket it will take time to wear it down so your portafilter once again aligns at the 6 o'clock position.