Personally I use Chevron DELO straight 30w, but for cold weather I'd use DELO 15w-40. I'm a locomotive engineer and at Los Angeles' yard's diesel service facility they have a 10,000 gallon tank of DELO 30w. DELO = Diesel Engine Lubricating Oil, and if it's good enough for 4,500h.p. V-16s, then it oughta be good enough for my little straight 6. Costco usually has the best price on a case of 6 gallons.
All the major oil companies formulate a diesel engine specific motor oil, and unless I saw results from direct comparison tests using SAE/API standard testing showing otherwise, I would believe that they're all essentially the same. The specific brand you choose is a matter of personal choice / experience / availability. These are all 15w-40, heavy duty diesel engine oils:
Shell = Rotella
Texaco = URSA
Chevron = DELO
Valvoline = Premium Blue
Exxon = XD-3 Extra
Mobil = Delvac
Synthetics - Red Line, AmsOil, Royal Purple, etc. - may be better, but I prefer to use regular dinosaur juice and change it every 12-18 months or so.
Regards,
Jon