High pressure water won't hurt a thing, or once you get the engine running, you can overfill the crankcase with 2-3 quarts of kerosene or Diesel fuel after filling it to the normal level with 30 weight oil, and let the connecting rods and the crankshaft counterweights act like the agitator in a washing machine. The high speed splash will do a better job of scrubbing than any kind of manual brushing. Run the engine at fast idle for 30 minutes, 1500 RPM or so, then drain the pan while the engine is still hot. Just don't drive the truck or put any load on the engine while the thinned-out oil is in there.
Jerry
