Usually I dump it at Autozone where it is recycled. They used to take oil filters, too, but stopped doing that for some reason. So I've got a collection of used oil filters building up until I find another place to take them.
It's illegal for any car manufacturer to void a warranty if you do your own oil changes. They would have to prove that you did it wrong. Burden of proof would be on them. Modifications, well that's a different story....Dealership will still do an oil change even if its before the recommended interval. It will count against your "free oil changes" but having the dealer do it will prevent any possibility of warranty issues down the road.