Well, I put a little Permatex on it just to be on the safe side. The cover is an aftermarket chrome cover so it may not have the best fill plug on it. I had my Dana all gone through and assembled the summer before last at the local ring and pinion shop. It had just been sitting on the garage floor until recently. I finally got around to installing it in the car and then filled it just before my original post. Here I was worried about the fluid leaking out the fill plug only to find out once I put the new fluid in it is leaking pretty good out of the pinion seal (which was replaced when it was assembled).