You gotta stop unhitching it in the ditch.
Is this a Bush Hog brand or John Deere rotary mower? There is no drain. You need to turn the mower on its side. If it's a John Deere, you have likely reached the end of warranty and it's time to purchase a new one
I usually throw a bucket over the gearbox for the winter, if I remember. Funny how the Bush Hog can never find room at the inn under cover.
It's a Deere MX7. Beautiful mower otherwise.
At first, I thought it was rain getting in the breather. So I tried a bucket. Nope, not rain.
Putting that mower on its side is easy enough but isn't enough. It has to be upside down. I did that a few times. It's one very heavy, very awkward beast. Getting a drain pan in the right place is a fiasco. May do that yet again to get all the old oil out. If only Deere did a drain.