That looks too small, too thick, and too hard, it's not ideal for most people. 4mm ID will be very tight, so I'm not sure if you can even install it on the cap stems. 2.5mm thickness might be too thick, the cap might bottom out before the actuation point. The max thickness I used is around 2.3mm. And 70A hardness is much harder than 40-50A that most people get, so it's probably harder than most people want. Typical nitrile 70A rubber is pretty cheap (pricewise), the last group buy I did, it only costs ~$3 for a pack of 90 O-rings. It's when you get into other materials and softness in the 40A range is where things start to get expensive.
