Ours are rubber and were when we got the boat five years ago so that makes them best part of twenty years old and working well.
They are about 4mm thick and the bolt-holes, spaced every 75mm, are a tight fit for the bolts. No leaks in the past and didn't let water in when submerged for a couple of hours.
I think you want nitrile. If I remember it correctly neophrene is attacked by diesel, nitrile isn't. I have nitrile gaskets on my diesel tank and they have lasted ten years and are still sound. I.m not sure want is meant by rubber these days.
