The answer would be depending on temperature, pressure, ball-valve size and ball-valve construction. And: not at least fluid. Crude oil could be anything from fairly refined, clean and free from particles and easily flowing at normal surrounding temperature to something you could walk on, filled by sand, and requiring 100 deg centigrade or more to flow.
Nylon is a trade-name. My first choice would have been carbon reinforced PTFE (Trade-name'Teflon') for normal, clean crudeoils at normal processing temperatures, moderate diameters and pressure classes.
rak29,
for a first, quick reference, the elastomers' Manufacturers usually provide for chemical compatibility, mechanical and thermal resistance data under form of tables, software, etc...
See, for example, the "Chemical Resistance Guide" by DuPont Performance Elastomers (that also own the Viton ® and Kalrez ® trademarks...):
