Synthetic GL-5 gear lube should work just fine in the Duo Prop drive. Typically if there is an issue with GL-5, it's in an oder drive and it's with slippage, not sticking!
Your Duo Prop does not use a slinger pump.
The s/p drives must use Engine Oil viscosity due to the die-cast aluminum centrifugal pump (aka slinger pump) that is attached to the lower unit "driven" gear.
Gear oil in an s/p drive may damage the slinger pump drive pin. If we loose that slinger pump, we've lost upper unit cooling.
Either drive pumps cooler lower unit oil to the upper unit, and it then gravity drains back down.
Kevin, lapping is pretty easy, but I personally have a little trick that I use to ensure an even pressure pattern.
BTW, you'll be de-glazing the gear cups, not necessarily the sliding sleeve, but material is removed from both during the process.
You can also begin with fine emery paper on the cups.
I do this while rolling the gear along the edge of my rubber top work bench and while holding onto the bearing.
The emery cloth is applied to the cup while it's spinning.
This turns my hands and bench into a lathe..... so to speak. LOL
The main thing is "even-ness".
Back to lapping:
Again, I need to be clear on this... Volvo Penta does not say anything about lapping these, thus no suggestion as to a procedure..... at least none that I am aware of.
Lapping has become a procedure outside of Volvo Penta, but it does work, and many of us do this.
Now, the kicker is the coated gear cups.... if yours are the coated type from using the prior steel sliding sleeve.
You'll know if these are coated because they will appear to be darkened and not a shinny bright carbon steel color.
You'd think that removing the coating would bring these back to being similar to the standard cups that are intended for the brass sliding sleeve.
So this is where we must be extra clear that you're on your own with regard to doing this.
Woud I give this a try??? Yes, I probably would because the only other option is a gear set change $$$$$$$$$$$$$$$$$ or back to another steel sliding sleeve!
Nut shelling this:
Start by picking out any metallic debris from the oiling groves in the sliding sleeve.
Look closely, as there may be a small amount of metallic debris here.
NOTE: the sleeve is directional in that there is a top and bottom to them. It will say "top" on the side that is intended to face UP. So make sure that the sleeve is facing into it's respective gear cup.
With a driven gear down on the bench (cup facing UP), and lapping compond wiped evely on the cup, I rotate the sliding sleeve XX degrees with my right hand while pushing downward into the cup.
With one completed stroke, I'm continusouly indexing the gear/cup XX degrees in the oposite direction with my left hand.
This is to ensure an even amount of pressure around the cup since this is being done by hand... not by machine.
As you do this, you'll feel a change in friction as the lapping compond breaks down into a more fine cutting ability.
At this point I pull them apart and look at the cup to see if the glazing is being cut away.
(don't forget that some can be cut away with the emery cloth)
You can add a bit light oil to the compound if you wish.
Repeat the process if necessary.
Clean all compound from each component..... and in particular from the oiling groves in the sleeve and at the deepest area of the cup.
Oil the surfaces before re-assembly.
I sure hope that this solves the problem. Not sure what else to suggest.
About all you can do is to give it a try.
Use caution at/near the dock. I can't imagine docking while I could not find neutral!
