The current thread on favorite valve oils has prompted me to write some thoughts. When I joined the Brass Band in 1948 no one used oil on their valves, saliva was the lubricant, we were told not to use oil. I think now that at the time the only light oil available was Singer Sewing Machine oil, much too heavy for valves.
I see the reasons for oiling valves thus, to provide a lubricating barrier between piston and casing to allow fast movement without wear, to wash detrius to the bottom of the casing, to provide a diversion during a rehearsal when we have a passage we do not want to play.
How Much? The volume of the space between piston and casing depending on fit and wear by measurement and calculation is around 1/60 milli liter or less, one drop is approximately 1/30 ml. or twice as much as is needed.
How Often? I apply 1 drop per valve every time I take a horn out of the case to play, at a long gig I endeavor to oil at least once during a break.
Oil is cheap, to oil only when the valves slow down is false economy, a valve rebuild is expensive.