I haven't changed my diff oil yet, trans oil was a fun one. If i get the money this christmas, I'll be fully replacing my ignition as well.
For seafoam, 1/3 in oil, gas tank, and vacuum line. For the vacuum line, pour it in SLOWLY, you'll hear your engine have increased load and then you'll see the white smoke pour out....like this -
http://www.youtube.com/watch?v=8ffSDy4D-zg - lol.