Shifting, I suggest you start out at the 375cc. If you find it is bottoming, add some oil throught the air bleed screws at the tip of the fork. You can safely run 400cc.
As for oil weight, it is a topic of some discussion, but I'd stick with something below 10wt. With the heavier oil you will find that you have to run your clickers lighter in order to get the same ride as with the lighter oil...but, you will also find each click, with the heavier oil, is a great range of adjustment (if that made any sense at all).
Too much oil will hydraulically lock the fork, reducing travel.