Outboard lower unit oil color question

I have a two year old 60 HP Mercury outboard. The oil color in the lower
unit is a mystery to me. After the first three months, I drained the oil.
It had a nice, honey brown color, with some metal flaking. I refilled it
with Mercury gear oil. Nine months later, I drained it again. It has a
nice honey brown color with less metal flaking. This time I filled it with
Red Line Synthetic 75W90 gear oil. Nine months later, I drained it. This
time, it was inky black. So I filled again and 8 weeks later (being today),
I drained it and it was inky black again. I probably had the boat out maybe
10 times at most over the past eight weeks.

My understanding is that inky black oil means that it is probably old and
has not been changed for a while. That is not my situation. I do use this
outboard for duck hunting and it has seen some single digit temperatures on
the water. I have been careful to leave the motor submerged and upon
getting out of the lake, to tilt the motor to make sure the water has
drained.

I talked to both Tracker and Red Line and neither has come up with an
explanation for the color. They said if there was a problem, I would know
it. The motor seems fine though.

Here are some theories:
- The cold weather has blown a seal or cracked something where perhaps the
exhaust is getting into the lower unit. One problem with this theory is
that there is no moisture in the oil. It also seems that the lower unit
when I went to open the top two screws would have gushed out because of
extra stuff being added to the oil. That was not the case either. Is it
possible for exhaust to leak into the lower unit? What about two-cycle oil
some how getting into the lower unit?

- The Red Line oil is reacting with the aluminum housing and produced
aluminum oxide. Red Line's chemist says this would not happen. Tracker
says that they have seen different types of oil produce different
colorations including black.

- The Red Line oil reacted with residual Mercury Gear Oil when I first did
the change. That's possible, but seems like the second change with Red Line
would have eliminated that problem because the old oil should have been
completely flushed.

Anyone else have experience with this?

Jeff