Everyone has an opinion on this.
Your grandpa is right, but there are some caveats. If the engine has an oil filter, either bypass or full-flow, it is okay to use detergent oil. As Stuart said, the detergent keeps the dirt suspended in the oil, requiring a filter. Otherwise the dirt continues to pass through the bearings. With non-detergent oil the dirt settles to the bottom of the crankcase, more when the oil cools off. This is also why it is important to do a non-detergent oil change with the engine hot.
If the truck were mine and it did not have a filter, I would run non-detergent oil. Around here you can buy it at a parts store or farm store. If the truck has a filter, I would run detergent oil, as long as I could do it without grandpa finding out. Some things aren't worth the argument.
