Hi Antti
When oiling valves, unscrew the top and pull the valve halfway to two-thirds out of the casing. Next, put two to three oil drops on the top of the valve’s piston. Place 2 or 3 drops on the valve’s other side Push the valve back in its casing. Do not twist the valve while pulling it out or pushing it back in. The valve should not have to be forced in, it should gently slide into place. You may have to give the valve a slight left and right twist to seat into place.
Do not oil from the bottom through the hole on the underside. If you do, the oil will wash particles and grime onto valves.
Here is a Yamaha maintenance video: https://www.youtube.com/watch?v=cH1ak_kkZSE
Another maintenance video: Bhttps://www.youtube.com/watch?v=oI5eyUrIoIE
Happy Trumpet Playing,
Ronald