Here is what I would do if it was my lower unit. Since it has oil of an unknown type and age, I would drain it as completely as possible. I would refill it with kerosene and let it sit for a couple of hours. If I had the lower unit of to replace the impellor I would shake the lower unit to help remove the buildup of thick goo on the inards of the unit. Then I would drain the kerosene and let the unit dry overnite. The next day a I would refill the unit with a good quality lower unit lubricant. If one of the plugs is magnitized or has a magnet mounted on it, it should go in the bottom hole.
I would check the oil that was drained out of the unit, and the kerosene that I flushed it out with using a magnet to see if there are chunks of metal that would indicate the unit should be torn down and parts visually examined or replaced.
Just my opinion. I realize opinions are like noses. Everybody has one. Some are cuter than others and some just get in the way.