I guess I am the odd one of the bunch, I use organic olive oil I get from Sams. It adds good flavor and helps the meat to hold together as well as not stick to the pan when cooking. I usually just dump a bunch in with my meat, if I had to guess, 4 to 5 cups per 20-25 lbs. Main way i check is fry up a patty and see how it cooks.
My dad started doing it years ago, said it was cheaper and better for you. He always said that it didn’t make sense to him to add domesticated animal fat to high quality organic venison. As farmer he said that the chemicals, heavy metals, hormones and other crap that makes meat non-organic settles mostly in the fat of that animal. By mixing it in you are getting a higher concentrations of it. I will be honest, I never fact checked his claims, so don’t take this as gospel. It made sense to me and I enjoyed the flavor and so I have carried the practice on with my family.