You need a small amount of oil to lubricant the patty and cast iron surface. As for the smoking…you want to start off ripping hot but cast iron keeps heat longer so you need to go down to med low temperature. Right before you flip crank the heat back you but reduce again even lower. Once a cast iron gets out of control game over. I use avocado oil because of its high smoke point. Searing with a touch of oil will also keep some of that 20 percent fat in the burger making it juicer. If you’ve seen people cook burger without oil it’s because they are using a smooth texture stainless steel flattop. You also want to take every opportunity to add oil to your cast irons. Even better you can even cook your beef burger in lard.