Most oils (including SUNFLWER!) hydrogenate easily on heating - that means they become like saturated fats, except unlike saturated fats that your body can deal with if you don't over-eat them, you have no process to remove them from your body.
Olive oil is one of the best, but still does change - but the very best oil ever is coconut, which is in fact solid at room temperature. It is a mid-chain fat, the only one, and there is evidence to show it does not get processed by the body in the same way as asaturated fat. It does not change its structure on heating.
The only probelm is some people do not like coconut - we've got used to it!
Even better is tto use no oil at all and if you have to fry, do it in water or in the meat's own fat. Or grill!