Varnish problem in air valve

The oil particles in compressed air from truck decomposed and forms varnish inside the solenoid valve assembly after certain period of operation of valve. Due to this pluger gets sticked to sleeve which fails the operation of valve. It would be highly appreciated if you could help me on following,
a) How varnish created in air valves?
b) How to test our valve design for varnish ?
c) How to measure oil in compressed air from a truck?