I use a method somewhat different than Plasticbattle, altough it uses the same basic premise. I use all acrylics for mine. I start with a layer of light tan. As with Plasticbattle, perfect coverage won't be necessary. Then I layer on a reddish brown, light grey, and a darker brown using drybrushing with the brush being drug along the putative grain of the plastic. The order in which you apply the colors can be varried to get different colors of wood. If you accidentaly apply too much paint, resulting in a smooth even coat obliterating those beneath it, don't panic - just run a drier brush through it in the direction of the grain until you've got enough empty streaks for the older paint to show through.
Here's an example:
HTH. At least it's an option if you don't have any oils lying around, or don't want to wait for oil's long drying time.