Hey Stretch. I built that Star destroyer kit too. haven't painted it yet though. When I do, I'll definitely use washes.
Oils are good. There are some good tutorials on YouTube for how to make and use oil washes, i'd recommend you look there.
Second, don't use straight black - it's too stark. Try mixing black and a variation of the base color (medium grey) at about a 50/50 ratio. That will give you some darker places around the raised details, without making the model look like it's been dipped in a black wash.
I don't know how you painted it (the images don't show) but if using an airbrush you can get quite good results w/ post-shading and highlighting - this is sometimes called Color Modulation. Back when I started doing it we just called it shading.
You can also get some nice effects by mixing different shades of color into the wash - some blue for a cool effect, some yelow for a warm one.
In all cases, it's important to remember that the wash should be very thin - about one part paint for 20 parts thinner. It's better to under do it than over do it. You can easily add more paint, in thin layers - taking paint off is a real pain.
Helpful?