That would leave quite a bit of an oil layer on your gas tank's inside.. no wonder you're burning oil ! Theres not much you can do except draining your tank, use that gas mixed with oil for your lawn mower or something and putting in fresh gas, and maybe a bit of seafoam just to clean it a bit, though I would think gas alone would clean that remaining oil...Your tank wouldn't have rusted if you filled it with gas all the way to the top..after all..gas IS oil..
You might risk gunking up the cats, I would drain your gas tank at least once and put in fresh gas again.