Oil is a good idea on white woods, seems not to be necessary on colored woods since they have more oil in the wood to begin with. You would use the high setting on your gun, and mkeep it moving and heat sections of around a foot at a time. Feel the wood with your hand, when it is so hot you can't hold your hand on it for more than a moment, it is hot enough. Sometimes I'll clamp and then heat the section a little more just to be sure.
I've also clamped the ends of a blank and heated a whole limb at a time, it takes longer but when it gets good and hot you can see the limb realx a bit and take on the shape of the form. At that point you can clamp it along the length.
You want to hold the gun a couple inches away from the wood, and have patience. You want the heat to penetrate through the blank. I would never heat the back itself though, because heating tends to make the wood slightly more brittle and that is certainly not what you want for your back.
Get you blank down to an inch thick or less, the closer to floor tillerred the better. This will reduce the time it takes tom properly heat the blank, and it reduces the stress on the limbs as you clamp it to your form. I've actually chrysalled the backs of a couple of bows by trying to bend them too far towards reflex. Some whoop-de-do's are best corrected in modest increments, and some you never can get totally straight but thats the beauty of selfbows.
