Hi taitny,
OSC works sort of this way, remember the old axiom "speed costs money, how fast do you want to go" ? Sort the same here, "Speed costs energy, how fast do you want to go"?
setting the OSC to XT gives the OSC less power to play with than HS does. and the settings that allow 32KC OCS give even less, generally you want HS if you go over 4mhz.