Is it leaking where the two halves come together or out the top by the electrical connection? If it looks like it is down near the threads, it is more likely to be the "O" rings for the filter adapter and what you are seeing is seepage from there.
I always use an open end wrench, rock it a little before I try to remove it. *Some* have what look like a place to unscrew it with a large wrench on the pot part of the sender, I've never had any luck there. I always use an open end on the fitting near the base. There are several different models of the sender I've seen. each with a slightly different setup to remove them, most all have a nut type fitting near the base. I always try it by hand first, one time in four you can unscrew it by hand, be careful not to twist the top of the two halves or snap it off.