Water condensates into the case over time, just builds up if the fluid isn't changed in like, forever. It has an open breather, after all.
If it was me, since I'm a cheapskate and don't like tearing into the tranny, I'd hunt down four quarts of 10w30, SG grade if I could but that's rare, plus two quarts of the fluid I'm leaving in it (Honda MTF for me) and:
-drain the fluid and fill back up with 2 qt 10w30
-drive 5 to 10 miles
-drain the fluid and fill back up with 2 qt 10w30
-drive 50 miles
-drain the fluid and fill back up with 2 qt Honda MTF
-drive about 1500 miles
-buy more MTF, drain fluid, refill again like before
-change again in 10-15k miles from then on