I have the phase contrast setup for my AO 20, but I don't keep it set up for it. I just discovered this sub-forum for some reason! AO microscopes are a passion of mine as well. I have refurbished a dozen or so to stock my wife's classroom.
What I have struggled with is adapting a camera to it. The only seller of the adapter extension that I have found is on ebay and he blocked me after I asked a question regarding the length of the adapter (and this was at least 8 years ago). I cobbled one together with pvc but it's ugly and only marginally functional..
Sorry to hijack, I got excited to see AO posts here.