Just a word of caution if you live in an area where they have emissions testing. A cooler thermostat may make it more difficult for your truck to pass because the engine won't get as hot to burn off all the offending gasses. I have a 160 degree thermostat in my truck and I have emissions testing where I live but, I am exempt from it .
What Sy said was right. And also without a programmer-module you'll will get a SES code for low temp. Newer 98+'s wont even allow you to do this with one. The newer 4.3's are looking for a temp reading of 170-200, anything lower will cause a detune on the 98+'s. And it has nothing to do with open and closed loop like many believe, that is controlled by exhaust temps maintaining above 600 degrees at the 02 sensor's.
no my area doesnt require testing, they dont care to much either cause i used to have a toyota with no cat or muffler and i never got pulled over and it was the loudest thing i ever heard. so are you saying i shouldnt but on a lower thermostat without a programer? i have a tbi 4.3 vortec.