I use oils for all my washes; panellines, wheelwhells etc. They are also great for subtle fading of panels, drybrushing, exhaust stains, oilspills. Just about anything really. They'r awesome They take forever to dry thus making them perfect for all sorts of weathering.
For washes I dilute them with mineral spirits and apply just like any other wash. Because they take long time to dry I can work with them to get the desired effect.
Fading of panels or adding shadows, highlights on figs etc. I apply them undiluted and blend them into the finish using soft brushes of apropriate configuration. Again the long dry time makes it possible to get the desired effect in a relaxed manner.
Why not experiment a little? New "tools" are always fun ;P
/Carl