So does it matter if i use 5w-40 instead of 15w-40?
Can you tell me If Rotella t6 is also fine?
All of those options would be perfectly fine, including a 15w-40. Delvac 1 is probably most easily located at a distributor or a truck stop.
And no, Delvac 1 ESP 5w-40 is readily available in Canada; Delvac 1 5w-40 not so much. In fact, finding any pre-CJ-4 lubes up here is difficult.
There's nothing wrong with M1 TDT 5w-40. It's just that the Delvac stuff tends to have more builder approvals. It's very hard to find TDT outside of Walmart here, and it's ridiculously overpriced there.