I don't know about this truck specifically, however it usually means air is leaking into the suction side of the hydraulic system. If the pump is higher than the hydraulic tank, the oil can slowly drain down the hose and back into the tank when the truck sits overnight. This leak could be anyplace from the steel tube that goes into the tank to the hydraulic pump itself. After a few minutes of running, the oil in the tank will be foamy on top if the hydraulic system is pulling in air. It should flow through the tank fairly clear. Also, the hourmeter means nothing. People regularily change hourmeters when selling a truck. On most trucks, this is easy to do.