Mercruiser Gear Lube

I have a friend who has an Alpha One sterndrive. A few hours after the last two services the alarm has gone off to say that the level of the gear lube is low.
My assumption is that they have replaced the oil, but just not topped it up far enough &/or that there were air bubbles that have worked their way out...

My question - does the lube oil bottle have anything to do with the lower gearbox, or is it only connected to the upper transfer case?
And if there is a leak in one of the upper seals, how can you tell?

 

The gear lube flows between the upper and lower units through the shaft and a return hole.

I've seen the gear lube monitor bottle filled differently to try and prevent air or planning to top off after the air comes out. Everyone starts by opening the vent screw in the upper unit, and then pump lube in from the fill hole in the bottom of the lower unit's bullet until it starts coming out the vent hole. Then I've seen people fill the lube bottle with the vent hole open until it stats to come out and then quickly put the screw in. Others have filled the drive as if there were no lube monitor (pump oil in the bottom fill hole, then put screw in vent hole, then put screw in fill hole, then fill lube bottle and wait for air to come out, then top off a couple hours later) There is only one small hose between the lube bottle and the drive so air can cause it to drop some -- I've seen it drop 1/2 inch but not totally empty ever.

Does the boat have a trailer? Can you let a little bit of the gear lube out into a glass bottle to see if it is milky or has any water in it? Whenever I've needed to have seals replaced, I've been able to see where the lube was coming out when it's on the trailer overnight around the prop shaft seal or wherever. Rarely, it will only leak in places like the shift shaft seal when it's hot and under pressure or with a particular movement. That's rare.

The sure test is to have the shop do a pressure test on the drive using (low) air pressure and a gauge to see if it holds over a few minutes. If not, use soap and water to see which seal bubbles.

 

Thanks IMP-ish.... so there is a link between the two...
I had him check the lower gearcase for water in the oil (none) but then later wondered if there was indeed a connection between it and the monitor bottle....
Out of interest, is it the same with all Mercruiser drives? (I have a Bravo 3)

 

Sorry, got busy with work and didn't have a chance to logon til now.
How did this turn out?

I've never had a bravo3. I assume it's the same but I'm not sure. All the bravos circulate much more lube in the drive (slightly over 3 quarts) (alphas take only over 1 quart; takes me a bottle and 1/3). I suspect the bravos have larger lube passages so less air bubbles... don't know if it works that way. It hasn't been an issue for me. Curious if it was the issue here or if a seal was letting go under heat/pressure underway.

When merc first started putting lube monitors on alphas (also had retrofit bottles for older alphas) I thought it would be nice to have a way to circulate lube through the monitor to show you instantly if a propshaft seal were letting water in. (had a lot of propshaft seals fail for a while. Then I actually caught the neighbor's kids fishing around my transom!!!) I think in real-world use the positive pressure alone helps a lot. And my idea of circulating must be impractical: more things that could break and lose lube at the wrong time. The simple "turtle flow" system would only dump the monitor lube in your bilge but not empty the drive I assume.