Not uncommon. We recently went through this on JCastle's RR with a 505. Think he had it apart 3 or 4 times finally the last time it was good.

Search for previous posts on this subject. Several good, long discussions.

His did not leak too bad right away either.

But a couple of points:

Make sure that you are not building pressure inside the motor - make sure you have adequate breathers. Don't overlook this.

In the back of the seal retainer (by the flywheel or TC) make sure you clean that area and fill it with RTV.

On the RR it was not hard to pull the pan and get to the seal with the motor in the car. We fortunately were able to work on it on a lift.

And good luck.........