Tips and techniques for using oil paints?

I'm not really sure about the water based oils. I don't know how they act and that's not the type I'd buy, but they make work fine. Always buy the best quality Professional grade oils and use Turpenoid or Humbrol Enamel thinners. (My 2 favorites anyway). They work great for pin washes. Use a dab of oil color and add around 75% or more thinner and apply it with a small brush in the panel lines, and wipe away to your liking. You can also place a small dab of oil paint right on the model on a spot you want to weather or streak, let it dry for a few min. they start wiping it away to create streaks or stains. You can actually apply lots of different oil colors in small dots right on the model and blend them together with a damp brush or paper towel to create fading and streaking effects. The possibilities are almost endless. Oil paints are also really great for drybrushing as they don't dry out on the bristles very fast. You Tube and google will help you out probably better than I can type though.

I started using Burnt Umber as a rust effect. I looks just like the rusted metal under chipped paint. You can also spray it for exhaust stains. I just starting using oils myself and am very happy with the results.

I use oil paints for washes, pin washes, and filters.  There's a ton of resources about how to use oils in modeling on the Internet.  Just google it.