Sorry to hear about the valve difficulty. I will assume that you are pressing the valves straight up and down — that your fingers are not leaning on the valves and pulling them (to the side) as you finger notes. Also that you are doing that finger action quickly/firmly.
Can you return to the store where you purchased the instrument and show them what is happening in-person? Perhaps they have a brass repair specialist who can remedy the issue? If not, as suggested above, contact Yamaha.
Related to when to oil valves, I oil after my last playing for the day to coat the valves with oil and get the other moisture off of them. I don’t have any issue picking up my horn the next day and playing without pre-oiling. If I’m using a horn I haven’t played for a couple of days I will pre-oil.
Hope things get sorted out for you and your trumpet. Cheers,