i've seen both type, actually had to use neither. right now i put an internal type in my
oil tank because it had the fitting and i had the element from another tank , the element looks to be made of brass about 1/4" dia.with the threaded plug portion brass also, pretty stout, i think if you burn through the element something is wrong big time. only problem i can see is if you plug the element in without it being immersed in
oil. probably pulls some pretty good juice also.
correction: the element is made of copper, the plug is brass.