I've never understood "which" and "that". I think of them as interchangeable - but I must use them differently, because WORD keeps on telling me I've doing it wrong, and I always find that the way it wants me to put it feels wrong, so I don't.

Spellings aside, I'm not sure there really is much formal difference between American and English usage, it's more at an individual level, and there are fashions, but I don't think they run along national lines. For example, I tend to prefer using commas pretty extensively. I noticed this when I was reading Lynn Truss's book.

One place where there a national difference though is when it comes to using "one" as a pronoun. In English English it is never right to use any other pronoun along with it. "If one climbs a mountain he can see a long way" can never be right - but I believe in American it could be. But I rather doubt if that would be likely to crop up in a technical manual. (The odd thing is that, if the sentence was "If someone climbs a mountain they can see a long way", that would be fine, but neither "one can" nor "he can" would sound right.