Like Parker says, front or rear. I'm assuming the front seal, it's possible that the bell housing is not centered with the crankshaft. a lot of people never check the runout of the bellhousing with the crankshaft, that's one of the reasons they sell offset dowel pins to correct the problem. It's usually a converter problem, the hub is not concentric with the converter. If it's more than .005 there's a good chance it will leak when it's parked. been there and done that.
JMO
Zip.