That's a real common find, in fact finding one without water in the transmission is rare.
Before draining it, do some diagnostics.
Open the inspection covers, raise the lift with a load, look inside and see where the oil is going.
If it's coming down out of the cylinder, the seals are bad, possibly the cylinder too, but it might hone out.
There is a stand pipe that brings oil up from the pump to the cylinder. There are orings at each end, one may be blown.
There is a pressure relief valve on the pump. If it is leaking oil will be swirling beside the pump. Raise it up, shut off the engine, look for the swirling.
Just don't reach in there with it running!
All that can be repaired without removing the pump.
If the lift raises fast and steady, the pump is good. If the motion is jerky and slow, the pump valves have failed and it will need to be repaired or replaced. I recommend replacing, they have close tolerances, are difficult to repair and replacement parts don't always fit correctly.
After diagnosis, then drain the oil, rinse it out with diesel or kerosene if desired, not required.
I like to refill through the inspection cover holes. It fills faster, no chance of overfilling or flooding the clutch with oil, as can happen when filling through the fill hole. As for the oil, 15w-40 motor oil is a common substitute. The 90 weight mineral oil (not to be confused with 90w gear oil) will work, but will be slow to operate the hydraulics in cold weather.
Water tends to get in the cases through bad shifter boots, and the boot over the top link under the seat.