This is just a guess as I am not a boat mechanic, so here goes. I would
have to agree with Bob L. as for the heat factor to turn the gear lub
dark or black in such a short time. Grab the prop shaft and see if you
can move it back and forth and wiggle it up and down. Maybe the bearing
carrier or the bearings are bad and the shaft is not lining up with the
forward or reverse gear and this is causing heat to darken the oil. I
don't think the oil from the power head could leak into the lower unit,
also the exhaust gasses would cause alot of blow by into the lower unit
as they are under pressure, so I don't think this is the problem either.
Next time you have the boat in the water, run it for a few miles and
when you get back to the dock go from forward to reverse a few times.
Shut the motor off and bring the motor up in the power trim and you got
to be fast as the water will cool it off and see if you can feel any
heat transmitting from the gear case. You can always go back to the
orginal gear lub and see if the problem still there. Let us know how you
do on this situation as I would like to know. Thanks and good luck.....

sket