I think it depends on the engine you are using, but for UDK I'd make the broken parts and oil/blood spills into decals. So you have a clean base material and can decide where to put the decals. Sometimes they are a nice option to cover seams between tiled surfaces and further break up the monotony. Decals should be handled…
Hi andycain- I made this tutorial on tiling texture creation. I start with photoref and then process it quite a bit. I also create a normal map based on the diffuse. Not sure if this is the kind of thing you're looking for but I hope it helps!