The trans fill plug is tapered like a pipe fitting and the diff plug is not. Don't overtighten the fill plug or you can crack the case. It's a good idea to make sure you can get the fill plug loose before draining because you are stuck if you drain it then can't refill. Teflon tape is a lot less messy than any kind of putty but I didn't use anything. The fill plug on my car was square drive but a 16mm fit.