Hi JT, 5 ounces does seem like a lot for 1 season. It is not an engine so it can't "burn" it out
There really are only a few places it can leak out though and I would think you would see it after 5 ounces.
It can leak out of the prop shaft seal, lower housing gasket or upper housing gasket. With any of these you should see a raibow "sheen" in the water maybe just after running or after it has been sitting for a bit.
Could also be leaking at the input shaft seal (ask me how I know this one). This is the drive shaft with u-joints that connect the engine to the outdrive. Now when this one leaks, it will slowly fill up the drive shaft bellows and eventually overflow into the bilge. I found mine leaked worse with the outdrive trimmed all the way up, when down it only leaked while running. So if you bellows is in good shape it will not leak outside into the water.
Also could be the reservoir system leaking. There is the hose and fittings at the bottle and transom so it could leak at any of these points too. These should also leak into the bilge (or somewhere on top of the engine) so it should be visible somewhere.
So have you checked the bilge to see if it has leaked down there? After 5 ounces you should see something if it were leaking into this area. Also, are you checking the level with the outdrive in the full Down position each time? I have noticed my level varies a bit depending on drive position.