improving my ride as well as mpg...I hope

I just bought a used 2000 xe 4cyclinder. I've had it for about a month or so. I found out so far that the gas mileage could use a little improvement. Maybe a lot . My boyfriend wants to change the stock filter to K&N. But from reading the thread, I would prefer one that can be cleaned with just air. Plus I live in san diego beach area, so there is a lot of dirt particles in the air. I"m switching the oil to all synthetic. What type of oil should I use? It does get pretty warm around here most of the year. The shifting seems a little rough too. What would you guys suggest? Anything else I should do? There is a little over 115K on it so far. I'm also getting all new stock tires from bfg. I read earlier it is best to have them at 28 psi, but then read later to have it close to tire recommendation for best ride and mileage. Which is it?