I perfer oils. They're easily blended into the base coat and stay workable for hours. Use artist quality turpentine to thin the oils. If using enamels I would recommend using a clear, flat laquer over the base coat to keep the turps from attacking it. Wait at least 2 days for the laquer to cure and harden. Personally, I don't use a clear coat, but I let the paint cure for a week before touching it. Lately I've just been using the oils straight from the tube for shadows and using a clean brush, a rag and Q-tips to to blend and remove the oils to bring out highlights and add heavy nice dark shadows.
I used the technique on this little project.
Notice the heavy shadows and the bright highlights from drybrushing 5 runs of progressively lighter shades of the base color. I mixed 1 part of Lamp Black oil to 3 parts Raw Umber oil. If you have any questions, ask away.
Jim