I have been using acrylic paints to weather my models for some time now. Lee Turner has convinced me that this is the way to go for creating a wide variety of effects.
Ammo MIG is major force in producing products for painting and weathering. Most of the market is oriented towards armor, aircraft and dioramas. They have both acrylic and enamel (oil) products in their line. MIG introduced a new way to use oil paints for weathering models. Each tube has its own fine brush that can add a spot or streak of color. The oil can be drawn out with mineral spirits. I tried out the MIG product on a piece of aluminum corrugated metal. I dabbed a few spots and used the mineral spirits to create a thin color wash. I finished by using a gray acrylic wash to blend the colors.
I was messing around with oil on wood. Those of you who modeled in wood using Floquil paint to rub a finish on. It appears that one can replicate the old school methods featured in Finelines decades ago.
I need to play around some more to get a better technique for application.
The real advantage of oil weathering is that you can apply it over acrylic paints without softening the base color. This is also true with most lacquer finishes. Jim Booth is a big fan of oil-based weathering. He recently finished up weathering over 100 Sn3 K-class imports. P-B-L has offered custom finishing by Jimmy for years. The picture shown below was taken by Jimmy after finishing up the locomotive. Much of his weathering is done with an airbrush. Mineral Spirits is the preferred thinner for oil. Jimmy suggested adding a drop of Japan Drier to the mineral spirits. It accelerates the drying process.