the ex500 uses a cloth type filter element that can be cleaned-reused. (at least mine does)
i clean it using wd-40, and spray it until it starts dripping black fluid (yum)
i squeegee it off, and let it air-dry for about 15 minutes. once it's dry, i take maybe half a capful of oil, maybe not even that much, and massage it into the surface of the filter, till it feels like it has oil evenly all over in it. put a little oil on the foam gasket to seal it to the airbox, and you're done.
if you use too much oil, you'll have a puddle of oil under your bike for a few days as it drains out of the filter/airbox.
as far as what type to use, i use whatever oil goes in my bike's engine, as i usually clean it when i change my oil. tha's also what's reccomended in the user's manual.