Well there are a number of ways to dirty it up.
1. Give it a wash. This does a good job of looking like oil.
2. drybrush it
3. use pastel dust. You can use a lot of different colors. just sand down some chalk pastel put it in a small pile, and then apply with a brush.
4. Give it some scratches. if it was used in war then it is gonna have some wear. I use a #2 pencil.
5. since it is a later war vehicle, and might have seen more combat, you can give it some broken fenders, or really banged up Shurzens ( Spelling
)
THere are probably a lot more ways to make it look dirtie and used I am still new. So somebady else will have to fill you in.
And
to the darkside!!!