Since I've got a few miles under the car and the fill plug was leaking, I decided to drop and replace the fluid in the diff.
The fluid came out looking none too worse for wear as you'd expect with only 600 miles on the car. Just the normal sorts of things you'd see.
The leaking occured because I'd over-filled the differential and reused a single-use-only sealing washer. When I filled the diff, I had the rear of the car on jackstands while the front was on the ground. The rake of the car allowed the diff to take more fluid than it should have, the level of fluid when the car was on the ground was above the fill plug. Between that and the expired sealing washer, it leaked.
Refilled the diff with the car level on QuickJacks this time, replaced the collapsing sealing washer with a fresh M20 copper crush washer and torqued the fill plug to spec.
- Previous fill - Motul 75W140
- Current fill - Royal Purple 75W140