I've seen several posts from people claiming that Mana Preservation I works on spells over 20th - contrary to what the description says. I've noticed though that these are all from people playing hybrid characters (i.e. rangers) casting thier spells that they've attained after 20th but that thier caster component gets earlier. One shaman player also claimed that it worked on greater healing - but that is a spell clerics get before 20th (if I remember correctly). I think this is what one must consider when figuring out which spells MP1 will work on.
Xlistin