The first issue when talking about pneumatics vs. hydraulics is always going to be severity of the duty. Obviously, the durometer of the seal is very important. When you get into a 70, 80, or 90 durometer seal, it becomes very resistant to being smushed. As the pressure increases to, say, 3,000, 4,000, 5,000—maybe even 10,000 psi or more, it just is a harder durometer. It’s more resistant to movement.
There’s also chemical compatibility to consider. Obviously, there is not a whole lot going on with air. Air is air. Sometimes it has some additives like oil or mist added for lubrication, but it’s still generally air—and most everything in the world is compatible with air. But for hydraulic fluids, they can change a lot based on applications. You can have arctic oils, you can have high water based fluids, you can fire resistant fluids, all kinds of fluids—and this is where the seal is critical to be picked for those applications.