Argon mix every time for car repair i'd say. Makes for a nicer weld. You can as said use other gas but this just makes it effortless and closer to a pro look weld imo. You will struggle to get this though without paying rental on the bottle and actually being a trader. Do a search though as I got a small knee high bottle mail ordered to my home address without need for rental etc. Can be refilled for 30 bucks too. Dont remember the company name but could find it if need be