The output flange on my port engine broke loose as the center bolt give in of metal fatigue stress (misaligned shaft). When the flange was pulled out all oil in the gearbox emptied itself into the bilge. This happened on our way into a narrow tidal lock in Holland. Ouch! This was on the 1800 nm get-home run from southern Portugal to Scandinavia with our to us new boat.
Look at that color! This is supposedly straight 30 weight oil.
At the time the gearboxes had 1.000 hrs/15yrs on them. Could be the oil was never changed since they were installed?
Sourced a new bolt from a nearby yard, installed the flange, stretched the new bolt to the specified torque (we do not go on a cruise without a torque wrench, do we?) and filled NEW oil. We were on our way again after a 2 day delay. No trouble since.