I'll just cut to the chase!
1) How often should I empty the spit valve? I am getting sort of compulsive about it, emptying it almost every minute, hahaha. But the thing is, it's always got a fair amount of spit in it. Is this because I'm a beginner? Will I use less spit for my notes as I develop better embouchure?
2) Is it okay to oil my valves only when I feel them beginning to stick? So far that's been only once. My trumpet isn't worth a Strad, of course, so is it okay to not oil it every day I play like I've read in some places?
3) Breathing question: What is the principle of 'stale air?' Should I empty out all of the air in my lungs before I inhale more? Or should I just breathe in more on top of what has already been in there? It's not like the breathing times are always perfectly in time with when I am completely out of breath. It just feels bad to inhale air when I still have air in me--I've been taught to exhale everything before inhaling in choir anyway.
4) Tuning slide. How do I use it? I don't understand, because I don't understand where the 'center' of the open C is. I can bend it to almost an A under the C, and can only go just barely sharp over the C. Should I move the slide so that I can bend equally in both directions from the C? With the slide all the way in, it still is much easier to be under the pitch than over.