If you are not very familiar with the standard types of seal available from stock then review the section on standard seal types available from manufacturers.
Reference should be made at every stage of the selection process to the Product Design Specification (PDS) for the system (all the relevant factors should be described in a well written PDS).
The characteristics of seals available "off the shelf" often constrain the design of the system components that require the seal so be prepared to change their design to match a manufacturer's seal.