I agree with Bill on the previous use and whether or not you have filter. It's true, these trucks were designed to run on a single weight, non-detergent oil.
Without a filter, I would run SAE 20 in the winter, SAE 30 in the summer in a non-detergent oil. The theory is that the dirt settles in the pan, then is changed hot so the sludge flows out when the oil is changed.
With a filter, I would run a multi-viscosity detergent oil, preferably 10W-30. Dirt stays suspended in the oil and is trapped in the filter. I do not like the 10W-40 oil, it has a lot more polymer in it to spread out the viscosity range. For years GM would void your warranty if you used 10W-40. I know the oil has improved, but it still has some problems.
From an engineering standpoint, running a multi-viscosity oil in an engine designed for straight oil will not cause any problems, as long as it is an oil that doesn't suffer from premature breakdown, like the 10W-40. You still have the lube quality of the higher number, but when the oil is cold it flows like the lower number due to the polymers added.
Another issue is cleaning the inside of the engine. Years of sludge buildup can cause problems, but cleaning it out can bring another issue, leaks. The big item is to keep the oil changed and to run the engine at normal operating temperature on a regular basis to keep the water boiled out of the oil, which is what causes the milky residue in the engine.
