Maybe my experience was a rare exception, but...the supercharger on my '92 SSE was a completely rebuilt unit from Magnuson. The nose oil seal began leaking at roughly 20,000 miles (that'* when I first noticed it), after the warranty had expired. I replaced the seal myself, and when I removed the pulley, I discovered that the seal was leaking because the inside edge of the pulley was rubbing against the outside surface of the seal, and had chewed-up the seal over time. They had either pressed the pulley too far onto the shaft, or pressed the shaft too far into the snout. When I reinstalled the pulley, I didn't press it on quite as far as it was before.
This supercharger has a bar code sticker on it, and the numbers begin with "AS".
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles