What's a good lube for hydraulic valve spools?

[DR Buck]

Wondering what is a good lube to use on top of a hydraulic joystick valve where the top of the spool sticks out?  I just rebuilt one of my log trailer joystick controls and noticed rust around the top of the spool where it sticks out above the valve body.  It's under the rubber bellows, but still had a fair amount of rust on it.   I'd like to cover it with some kind of oil or grease but I'm not sure what to use.

[North River Energy]

Mobil Vactra way oil.  It seems to stick to the iron despite repeated condensation cycles.