JM, I run 195 thermostats in all pick-up and passenger cars with 230-250 engines. The 292 engines in heavy trucks/ag equip. get a 180; even at that extended idle during the summer may require an auxillary fan. If you are fighting a sludge problem insure that the P.C.V. system is intact and correct for the engine. Each P.C.V. valve contains a metered orifice that is calibrated for specific apps: remember it is a closed loop system; most of the older stuff I work on is missing the fresh air connection to the breather; as long as the inlet air is filtered you are O.K. "Preach" nailed it: Sludge is a result of low temps and other related factors , varnish is high temp deposit...fats
