dndrich - brilliant. These are softer than the rubber ones - and, providing you do not damage them, will last a very long time - years! - before they need replacing, if ever.
I have one which is 4 years old, and it is as good as new.
If you find yourself having to over tighten the portafilter, then it’s likely the shower screen is screwed in too tight - just losen it up a bit, and press the gasket as water might have got trapped behind the gasket. I know it seems odd, but it does happen when the screw is in too tight.
And there’s no need to apply any grease. It’s silicone, and providing the surface of the group was clean when installed, it should not stick. And it definitely won’t bake in with heat, unlike rubber ones.