What type of immersion oil should I buy?
For the best image (where you lose the fewest photons and get less refraction), you want the immersion oil to have a refractive index as close to the refractive index of the coverslip you intend to use.  In my experience, this is not much of a consideration on the oil-side, but rather on the coverslip-side.  

There is little difference between type A and B oil, although for other types of oil see here.  If I were you, I would just use some type A.

For a lesson on the impact of immersion oil on optics, see this Zeiss website (similar sites for Nikon and other scope manufacturers exist too I'm sure!)