To oil a valve, unscrew the top valve cap completely. Remove the valve from the casing (in other words, pull it out). Use quite a bit of the oil on the outside of the valve core, near (but not in!) the port holes. (Along the red line indicated by the arrow on the photo.) There is no negative effect from over oiling the valve! You can't use too much -- don't be stingy! (Take care where the excess oil goes -- don't get it on your clothes, carpeting, or furniture.)
Then slide the valve back into the casing. Rotate it until it clicks into place. Check for proper alignment by blowing gently throught the leadpipe. If your air doesn't flow smoothly through the instrument, try lifting the valve slightly and rotating it another half-turn. Then, tighten the top valve cap again.
Press the valve rapidly a few times to work the oil in completely. If you are oiling your valves just after cleaning, be sure to use more oil than normal, since most cleaning agents remove all the oil from the pores in the metal. If your valve is still sticking, it is most likely dirty. Check for an article on Cleaning Piston Valves.
Please note: Do not oil the valve through the hole in the bottom valve cap. The oil does not always coat the valve core completely and most of it just runs back out of the valve cap when the instrument is lifted back into playing position. [END].