I run either 5w40 or 10w40. The first digit is the oil's lower temperature threshold. The two digit number is the viscosity. At least that was the information I've been given. The thicker oil will stick to moving parts better and provide better lubrication.
They're both viscosity numbers (Hence multi-weight oils). The engine has to work harder to pump heavy weight oil (Lower mpg's) and it also takes longer for oil to reach the top of the engine during a cold start.
The practical differences between a 10w30 and a 5w30 oil used in one of our trucks are almost nonexistent. On a cold start at any temperature below about 70 degrees Fahrenheit, you are going to be on or nearly on the oil pump's pressure relief valve until the oil temperature rises above around 100 degrees or so no matter what oil you use, zero weights included. Oil quite simply does not flow well at those temperatures. That's why oils have additive packages in them with things like ZDDP and MoS2. These are boundary lubricants that come into play when the oil cannot successfully form or maintain a film and create the hydrodynamic wedge necessary to prevent metal to metal contact. These boundary lubricants work themselves into the metal itself and help prevent scuffing and galling until the oil can flow. The weight of the oil has nothing to do with its ability to protect the engine, in this case. It's the additives that are doing the protecting.
While we're on the subject of viscosity, we'll clear a couple of other things up. Thicker oil does not mean better oil. It just means... thicker... oil.
The reason that you see engines, like some diesels, call for a 15W40 or a 5W40 is the ability of the 40 weight oil to maintain the viscosity that the engine designer wants at the higher oil temperatures that are typically seen in modern, turbocharged diesel engines. It can do this because... it's thicker! Of course! The higher weight means a higher viscosity at higher temperatures! Not all oils are created equal, though. There is such a thing as a "thick" 30 weight and a "thin" 40 weight, and vice versa. Here's a chart to give you an idea:
You must be registered for see images attach
Oil viscosity can and does have an effect on fuel economy. Why? Pumping losses, plain and simple. It takes more energy to pump a thicker fluid than a thinner fluid, given the same temperature and flow rate. Should you be concerned about this? No. I would say that it falls near the end of the list of things to consider when choosing an oil for your truck. These are heavy vehicles with (relatively) big engines and the aerodynamics of a hay barn. The tiny percentage of loss or gain in fuel economy is not going to make a noticeable difference in your fuel bills, so unless you're trying to hypermile your brick, worry not.
As for myself, I have run the following: Castrol GTX 5W30, Castrol GTX 10W30, Mobil 1 0W40, Rotella T6 5W40, and Mobil Delvac 1300 15W40. My conclusion? The 40 weight oils give me slightly higher oil pressure at operating temperature. I currently have 8.5 gallons of the Delvac that I got for free sitting in my garage, so I'll be running it for the forseeable. I haven't noticed any difference in fuel economy. The last long trip that I logged my mileage on I averaged 18.5 MPG driving almost exclusively highway. On 40 weight.
To the OP: My '97 Suburban's owners manual states that while 5W30 is the preferred oil for our trucks, and I quote: " You can use 10W30 if it's going to be 0 degree Fahrenheit or above." You should be fine.
