50/50 blend of kerosene and ATF. Let idle till up to operating temp, then slowly bring revs up to around 1500 to 2000 for just a few seconds to "sling" the cocktail around inside valve covers, lifter valley, etc. Drain, refill, repeat. Two or three times and even the cruddiest engines get respectably clean inside. ATF cleans well, and will add enough viscosity to the kerosene to protect wear surfaces.
