First thing is, are you co-washing or using a regular shampoo? Co-washing (Using conditioner to "wash" your hair), will leave your hair moisturized and replaces some of the natural oils that shampoo normally washes out.
Next thing, be sure you're using a conditioner. If you are, great. Now try leaving some or all of the conditioner in your hair instead of rinsing it all out. That will also help give it a moisturized/greasy look/feel.
Yes to oil if the aforementioned isn't enough. Olive oil rinses or leaving olive oil in hair can work. I've found coconut oil to be very heavy and gives a lot of shine. Gotta be careful w/using oils though. Have to be sure and use a sulfate-based shampoo every once in a while to remove the build-up it can cause. If that's too expensive to experiment with, you could try a basic pomade or gel.
Bed Head products are good too, though I don't know if they're all CG or not. They're designed to give you that "I haven't washed my hair in a couple days" look. There's a contour creme by them called Foxy Curls and it's thick and heavy. It's pricey too, but goes a long, loooooong way.