Are there any transmission experts out there?

On my Bayfield 25 with a Yanmar 1GM engine, the KM2-A transmission slips in
forward gear. The slippage is enough to cut speed down to about half what
it should be. At higher rpms the clutch often tries to grab but only
manages to do so very briefly but repeatedly, causing a loud juddering in
the drive. Occasionally it even engages properly at low rpm and stays
engaged, so the boat can get up to full speed with full throttle.

I've had the cable off the tranny lever and there is no doubt that the lever
is being pushed/pulled all the way to the ends of its travel, and the
springs in the cable/lever connector are applying force to the lever.

A manual shows a two-ended drive cone (probably with surface ridges to let
oil escape) on spiral splines on the output shaft to give it a self-servo
action to grab and hold the forward or reverse gear. I suspect that the
forward drive cone has the little ridges worn down and wider and it would
need a thinner oil to let it grab the conical bore in the gear better.

In my (1979?) owner's manual Yanmar recommends the same oil in the tranny as
in the engine, but I suspect that the latest motor oils have friction
modifiers that tend to interfere with the clutch engagement. Has anyone
tried switching from motor oil to automotive automatic transmission fluid to
make their clutches work better? Auto transmissions also have fluid
clutches, as well as lots of gears and bearings, so I would think the fluid
should be ideal for this application. It is recommended for at least one
Yanmar transmission.

Maybe I will have to bite the bullet and overhaul the tranny this winter,
but in the meantime any other ideas for an alternate diagnosis or a quick
fix would be welcome.

Frank Stokes