agreed 20w is too thick exspecialy to run in a engine, it dosnt have gear box oil the oil you put in the engine lubes all the internals, it wouldnt of seized up and you said you started it after so its not seized,
judging buy the pic of what broke id take off the back wheel a just make sure the part of the drum that broke off didnt fall inside the drum causing the back wheel to lock up
if you were riding and it just locked up im guessing it was from part of the brake falling off an a bit of it falling inside or maybe you accidently hit the kill switch also if the bike starts fine now maybe you couldnt kick start in when it stopped cause of compession i know when i first got mine and tried to start it again it was extremly hard to kick i thought mine seized up too cause of the compression