Adam --

I'd guess that normal operation of the tranny pumps around (and splashes around) a lotta oil, and the seal keeps it where it's supposed to be. And keeps the dirt and rainwater where IT'S supposed to be.

When the transmission is at rest (= off) the oil is in the pan and not splashing around. But be aware that you must elevate the REAR of the car when you pull the tailshaft housing. And even so, you might get a quart or two out -- so have a pan ready!

But your tranny holds, what -- about 10-12 quarts? Only a little will splash into your face, down your neck, down your arms, in your ears, hair, eyes, mouth, nose, down your belly, into your underwear. Yes it will, it happened to me. Did wonders for my disposition. :)

Wife commented on it and the smell.

Not kidding -- have a pan ready, and stand (or roll) clear. But only a little.