The fill point will not allow the level to go high enough to go out the seal. There is no "O" ring on the input fill, it is a tapered pipe plug on the LH side of the differential housing, there is no drain plug unless one has been added. There is however an "O" ring about 5" in diameter that surrounds the pinion support casting. That is the part that has 5 bolts holding it to the main differential casting and has the U joint yoke. It is quite possible to damage the "O" ring while installing the Pinion support into the differential casting. It is suggested to apply some grease on the "O" ring and surrounding area and carefully check it (the O ring) all the way around as the Pinion Support is slipped into the casting. Wipe the bottom of the housing so it is free of any
oil, check it again later and you will probably find that the seepage is coming at the parting line between the main casting. Newspaper and Baby Powder can be used to track leaks too.