How many miles are on the truck. One thing that is common on these is that if you go over to far on oil changes it will become low. All engines use oil, the thing is that at a resonable oil change interval the oil becomes contaminated with enough stuff to make it appear that it is still full. But on extended oil change intervals it will use more then can be replaced. I change at 3000 miles which some people laugh at but I dont care. I wouldnt go over 5000 on a 7.3 or 4000 on a 6.0. My opinion. I know individuals who may infact read this thread that beat the living hell out of there 7.3's and they dont use a bit. So if it runs good now I wouldnt worry about it.