Probably 5-40, I don't think there's a 20 available for diesels. After a lot of research I'm running Rotella T6 5-40 in mine year round. The only real downside I know of with running an oil with a large spread like 5-40, compared to 15-40, is that the additives are more compromised. But according to lots of used oil analysis, despite the 7.3 being hard on oil, that stuff does well even out to pretty high mile change intervals. I won't suggest going past 7k intervals just cuz, but the UOAs I've seen say that it's perfectly safe. The more it leaks or burns and has to be topped off the longer the oil will go. Even tho it turns black & scary looking, replacing even a small amount adds lots of life to the oil. Some big rigs have an on board oil change system that replaces about 2 of the 10-11 gallons of oil in the engine automatically and that small amount about doubles the change interval.