I agree with the pp's that either you are going to get stretch marks or you're not, and oil is probably not going to make a difference. But, who cares, because rubbing oil on yourself is a very soothing and loving thing to do for your baby and yourself, nevermind stretch marks. Last pregnancy, I bought a bottle of "Belly Oil" from the health food store, I forget the brand, but it was a nice herbal oil with an olive oil base. I have also seen "Belly Balm" that is more like a salve, that is also supposed to be good. This pregnancy, I'm going to make my own oil. It's going to be an infusion of herbs (calendula, lavender, rose petals, and chamomile) in almond oil. I just got the herbs last night and can't wait to start using it!
Incidentally, my last pregnancy, I faithfully rubbed the belly oil on my breasts and belly every night before bed, and I never got stretch marks. This pregnancy, I'm 24 weeks already and have not started using oil yet, and so far I don't have any stretch marks. I don't care if i get them or not though, I just want to use the oil cause it feels good!