Headlight Problems

[M31]

My passenger headlight is acting strange...DRL works fine...high beam OK but low beam won't come on just stays on DRL when I turn the switch to the second position. I've always assumed the DRL and low beam work off the same bulb but how can one work and not the other? I don't have litronic just the regular lights. I've been thinking of replacing with Sylvania Silverstar but my owners manual shows a separate bulb for low and high beam so does that mean I need four in total to do both sides? That could get pricey!