This should be stickied or in a FAQ or something.
I read an article a while back about the best oil to season your dutch oven with. I wish I knew where I read it but I'm sure if you use google you'll find it. Basically, he did a fairly scientific analysis on all the different kinds of oils and their ups and downs for seasoning cast iron. His results were that flax seed oil is the best although its expensive. His method was to burn off any existing seasoning and bring it to bare cast iron using the self cleaning feature of his oven in his kitchen, then while it was still warm he rubbed a very very light coating of oil onto it and baked it at 550 for an hour (I remember he said that if he could have gone higher without using the self cleaning thing again he would have). Flax oil has a really high smoke point, so it was safe to do that at such a high temp. After an hour of baking, he turned it off and let it set until it was cool enough to handle but still pretty warm, and repeated the seasoning process several more times, I can't remember how many times he did it but it was a lot more than I've ever done it for seasoning a dutch oven. He went from having a poorly kept pan to a grey metal pan stripped of any existing seasoning and other gunk to a perfect seasoned jet black pan that had excellent nonstick qualities. I'm too cheap to buy flax oil and my wife won't let me stink up the house with it so I haven't tried it yet, but I'm intrigued.