Drain the oil and refill with straight HD 30 weight motor oil. Add 2-3 quarts of Kerosene or Diesel fuel so the crankcase level is above the level of the big ends of the rods and the counterweights. Start the engine and run it at a moderately fast idle, maybe 1500 RPM, for 20 minutes or so. The splash from the crankshaft dipping into the oil will spray the solvent all inside the engine, including the bottoms of the pistons and the cylinder walls. It will do a better job of pressure-washing the inside of the engine than just about any kind of manual scrubbing can do.

Just don't drive the vehicle or put any kind of load on the engine. After 20-30 minutes, shut the engine off and drain the crankcase while the oil is still hot. Refill with fresh oil and change the filter element if the engine has one.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!