Did you ever check your coolant level??? It sucks that we don't have a coolant temp. gauge, but 240 on the oil at 58º is definitely a sign it overheated (mine barely gets past 210 even in triple-digit traffic).
It has to be coolant related, as your oil is full (it wouldn't matter if they put 5/30 or 10/60). The better question is since when are you low on coolant. Unfortunately we don't have a low coolant sensor on our reservoirs, so if you see no obvious leaks, it might have been that way since new; that's why I always check all fluid levels to make sure nothing is low. Good luck, and keep us posted.