Can't fill lower unit on 7.5

[wbeaton]

My buddy has a 7.5 Mercury I don't know what year it is, but it's likely 60's-70's says Kiekhaefer Mercury. He recently noticed that the lower unit is leaking oil. We tried to fill it with oil and we couldn't get it to take more than a few drops. It wouldn't take oil from the bottom or top suggesting that there is an obstruction. What could be causing the obstruction? We have both filled many lower units before including this one. Any ideas?

By the way, the lower unit is not seized. It will still push water.