Is it a hydraulic drive or a gear drive?
If it's a gear drive you could do this: Dump the oil and put in some diesel or kerosene. Then slosh the oil around by driving forward, stopping suddenly and then backwards. Don't move it far, diesel/kerosene doesn't lubricate well, but 10 feet won't hurt. Then run the lift up and down to clear the cylinder,if the trans and hydraulic lift use a common sump. Then drain the diesel/kero. Pour in some lube oil to flush the diesel/kero out and add your oil.
Another way is to use a pump up garden sprayer filled with diesel/kero and use the wand to wash it down inside and out the drain plug.
As for where it came from, check your gear shift levers rubber boots for cracks and replace them. Or it could be from temperature changes and high humidity. Damp air can still seep in.
If it's a hydraulic drive unit, you'll have to drain and refill, maybe a few times.
