440 Oil Weight
Brand: I run Mobil one in everything I own. Amsoil and Brad Penn are good too. My local machinist uses Brad Penn. Just make sure it has the right amount of Zinc. I'd spend the money on the correct oil than buying **** oil and dumping in snake oil additive.
Weight: I run 0w 40 in my current 440. It's a little heavy for me but I like the hot pressures I see especially at idle. I've got about a 10-20psi buffer over what I need. I could stand to run a thinner oil though and my machinist tells me I should run a thinner oil. Maybe I'm a little over zealous about seeing higher oil pressure #'s.
I'd ask your machinest what to run for weight and start there. If your pressure seems too high or too low adjust accordingly. as a reference my last dart I ran 20/50 and got the bare minimum for pressures but it had full grooved mains and a HV pump.