I just purchased a 1993 with 122k miles. Blueish smoke on start up and louder than usual tick. I was going to seafoam the tank. When taking fuel filter out the fuel was black. I filled with seafoam and truck ran great until old fuel was used again. Truck seems to have a slight miss. Did previous owner use oil with diesel? Truck runs alittle better every time. Tryn to get fuel cleared up before I take it in and try and advance timing or do injectors.