They did not do anything wrong in my opinion....

It seems weird that you could just add oil and it not keep leaking..sort of
like it was fixed.

If there is a huge dump of oil from around the oil filter area, there is a
big o-ring that goes above the oil cooler.  That deteriorates over time and
it leaks like a sieve when that happens.  But, that is not a procedure that
is part of every oil change, by any means.

An oil change on you or van consists of

oil filter
oil
cleaning sump screen
new paper gaskets for that screen
2 crush washers for the drain plug and the nut that holds the screen in.

Adam P
81  Westy "The Brick "
70 Single Cab "Whitey"
74 Beetle "Ol Yeller"
73 Transporter (STILL at paint shop)
1988 Vanagon Wolfsburg
1976 Transporter (New CA bus)
1974 412 "Goldmember"
Used Vanagon Parts for sale (mostly aircooled)