When you change the oil, should you also change the filter? I've read some of the posts and they said not to change the filter, but just the oil, why is that? Should I change the filter?

Always replace both - the only people whom I have seen advocate different intervals was Amsoil, and I thought they recommended a quicker change interval for the filter, rather than the oil... maybe the people making the recommendations have a secondary filter of some sort, and they are referring to that?

if anything, you'd change the filter without changing the oil (w/synthetic oil)

filters are cheap.