SX-M, got it, that is what I needed
These sterndrives use what they call a wet cone clutch. And it is a clutch that does wear out just like any car clutch would. "Wet" means it has to have gear oil surrounding it at all times or else it will slip, and if it slips bad enough, burn up, causing it to prematurely wear out.
There isn't any kind of adjustment issue here. The problem is either a gear lube problem. Or the clutch is burning out.
When I get these boats in the shop what I do is I drain the lower unit gear oil first and see what comes out of it. The sterndrive holds close to 2 quarts of lube. If the lube in the sterndrive is low, old, contaminated, or has water in it, or of the wrong type, that will all cause the clutch to slip. When I drain them, if I get about 2 quarts of lube, I will fill it back up, water test it, and see what happens. If it still slips the clutch is wearing out, and requires replacement. If I drain the oil and it's low, or empty, or has water in it. Then the sterndrive seals are leaking. And the drive actually needs to come off and be re-sealed. And the clutch would get inspected while it's all apart.
That is what the issue here is going to be. Either there is a lube problem, be it low, old, or has water in it. Or the clutch is wearing out.
Does that all make sense to you?