1998 Nissan Frontier Please Help!

Engine Performance problem
1998 Nissan Frontier 4 cyl Two Wheel Drive Manual 168k miles

Hi. First off, I know little to nothing about cars, but I have a serious problem. I drive a 1998 Nissan Frontier pickup, 4 cylinder 2wd. I recently moved to Oregon from AZ pulling a small trailer, which was a little harder on the truck than I had anticipated. Now I need to get it registered in OR, which you can't do unless you pass a low emmisions test. There's no way that's happening right now. When I start it, and when I idle wispy fumes come out of not only the exhaust pipe, but the hood as well. I looke through the hood, and found that the airbox had oil in it, and so did the front-most sparkplug chamber. It makes the truck hard to start, and also causes it to die if I take a sharp turn at low speeds, like when parking. I have no idea what the problem is, so any help is greatly appreciated.