180° Thermostat

Installing a colder thermostat will allow your engine to run cooler and a little richer (more gas).  The stock thermostat is 196° and the colder one will be 180° so it won't be that bad on your engine.  There are some downsides on running a colder thermostat.  You might loose some gas mileage and it will take a little longer to heat up your car in the winter.  If you run your engine too cold then it will eventually damage your piston rings because they run best when hot. 

The way a thermostat works is when your engine reaches a certain temperature the thermostat opens allowing coolant to enter the engine and cool the head/block.  When you install a colder thermostat it opens earlier allowing the engine's temperature to drop, then you are tricking your computer to send more fuel. 

*Here is a picture of a thermostat.