With seals for pneumatic application uses, one consideration to think about is the severeness of the duty.
Obviously, the durometer is very important. When you get into a 70, 80, or 90 durometer seal, it becomes very resistant to being smooshed. As the pressure increases—say 3,000, 4,000, 5,000, or maybe even 10,000 psi or more—it simply has a harder durometer, it’s more resistant. Then there’s chemical compatibility to take into consideration. Obviously, with air, there’s not a whole lot going on with air—air is air. Sometimes it has some additives like an oil or mist added for lubrication, but it’s still generally just air, and most everything in the world is compatible with air.
Hydraulic fluids can change a lot based on applications. You can have arctic oils, you can have high water based fluids, you can have fire resistant fluids, all kinds of fluids, and this is where the seal is critical to be picked for those applications.