If it is completely around the inside, most likely the rear main seal, and your flywheel slung the oil. Ad dusty conditions, and you get that buildup.
Tranny leaks usually show a clean streak down the face of the trans., or "washed' as it is referred to a lot. Usually this oil runs out the weep hole in the bottom of the bell housing/torque tube. There should be a cotter key in that hole. It lets the oil out, but keeps larger debris out. These can get plugged.. The idea behind the cotter key is to move, wobble, and keep the hole open.
If you've owned and operated the tractor for very long, guess you'll just have to ask yourself if it used a lot of oil. More so than just burning... Or..., if you've had to add to the trans./hyd quite often.
Here is a link to New Holland parts site.
New Holland - After sales service: assistance, original spare parts, manuals
Go to the middle paragraph, and click on parts & service. It wil open into a window, where you type in your model number. It will then open to a page that has anything with 4000 in the model number. Select 4000 3 cyl tractor.
It will then open to a list of the various components of the tractor. I assume you have the 6 speed tranny. Open that section, and there will be an exploded view of your trans. You can see what seals, and gaskets are where.
