Taco, look up what oil codes mean.
0w40 is not a 0 weight oil. it is a 40 weight oil. it has additives in it that make it flow and "act" like a 0w oil at very cold temperatures. this helps prevent the oil from getting too thick when its cold and improve cold weather startup protection/lubrication. at operating temperatures it acts like a normal 40w oil.
in the same breath, Rotella T6 5w40 will "act" like a 5 weight at cold temps and be a little thicker and less easy to pump. but be a 40 weight at operating temps. Rotella T6 is a thicker oil across the board than the M1 0w40, but thats just the nature of the oil. the M1 TDT 5w40 however is the direct competitor to the T6, and is said to be thicker still. I'd imagine most people run the T6 over the TDT mainly due to price and the T6 being plenty protection.
regardless, Im still running the break-in oil, which is the Royal Purple i listed above. according to RP's spec sheet, its a 10w30 convensional oil.