If the sender unit is pipe thread, (which is often the case) you really REALLY don't want to over tighten it. (Now how would I know that? :-0)
If it is the sender unit leaking, some Gasoila thread sealant on the threads should help. I'd hate to see you buy a whole can for one little leak though. If there is a company nearby who does any petroleum piping installs, such as a gasoline or petroleum trucker, a heating oil company, or a gas station supply company, they may be willing to give you a little dab. If you are within driving distance of NW CT, I would be happy to give you some. I could probably mail you a little dab too if you like. PM to me your mailing address if that's the case. The stuff we use is similar in appearance and consistency to glazier's putty, but perhaps a bit more gooey.
The alternative to Gasoila, teflon tape, is in my experience, usually ineffective at stopping high temperature petroleum leaks.
http://www.gasoila.com/index.php?path=main.html
