I just checked my oil and it was very low. This may be incorrect, since I did it on the sidestand. I read the FAQs, and I'm gonna go back out and recheck it properly in a bit. Either way, I'm probably going to need to add oil at some point. Since I just got the bike, I don't know what weight/brand oil is in it. I'm guessing its 10W-40, but I'm not sure. Say I'm wrong and it was actually 20W-50. Would adding oil of a different weight be bad?

Secondly, I read a bunch of the threads about oil on here and a lot of folks seem split on what weight to use. I'm in Los Angeles, so it's pretty warm year round. This makes me think that 20W-50 would be a good idea. For the folks in Socal, what have you found works best?

I would go ahead and completely change out the oil and filter since you don't know what's in it or how long it's been there.  The manual says 10w-40, so that's what I use 

10w-40.  What brand(s) shop did you buy the bike from?  If you bought from a Kawi/Suzi/Yamaha shop, I can almost guarantee they use Yamalube, because it is the least expensive to buy in bulk (I used to work at a shop like that).  Torco is very similar (Kawi puts their name on Torco bottles).  Castrol will mix with most non-synthetic oils due to cleaners added.