Something you might be running into is condensation.
On larger, (think vintage ag tractors) when you operate them, the gear box's get warm and the air vent allows the expanded air to escape.
When they cool off air re enters via the vent or other areas.
Air has moisture.
On a wet/damp day go look @ a crankcase/gear case and you will see "sweat"
That is on the inside too.
Every year I drain off water/condensation from the bottom of my collector RED and Green tractors via the bottom drain plugs.
Most times it is about a QUART from the 15 or so gallon gear reservoirs.
These tractors are inside an unheated building and some are not run/used for several years @ a time.
It is the nature of things.
Gear cases almost never get hot enough to boil off the condensation unlike an engine that does.
I doubt an arid place like Arizona/or desert would have much of a problem, but that is not the case in high humidity areas.
--Just my two cents.
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