I dont think you have anything to worry about. Sounds perfectly normal for a 170,000 mile engine. Mine smokes when I accelerate hard or go uphill. If your car is like mine, it is probably wound pretty tight at 68 MPH. Remember, we dont have overdrive. Im sure you just have normal injector nozzle and injection pump wear. I have noticed that mine smokes worse when I buy diesel at certain stations too. I get less smoke with Chevron deisel for some reason. Cliff