The way to fill oil in the lower unit is definitely to fill from the lower plug. You attach the pump to the lower plug and open the upper plug. Then you pump oil in until it flows out of the top plug with no air bubbles. Just pumping until you get the first flow out of the top is NOT enough. There is some waste but, take my word for it, this is the ONLY way to do it right. I have watched the authorized service guys do it this way.