I have a 1990 240SX. THe Thermostat has a temp stamped on it (760 degrees). My temp gage runs a bit over the half way mark. Is there a "colder" thermostat available that I can intsall?

Theres no way that is a 760 degree thermostat. It is probably 160 degrees, in which case it is a good temp. Other choices are (i believe), 180 and 196, so 160 would be the coolest.

You have to know what the ecu is expecting and what is the transition temperature at which the cold enrichment turns off is?

Generally 1990 wanted a minimum of 170F and a maximum of 194F reading from the coolant temp sensor. The oil temp is generally 20F hotter and must be kept in the 190F-210-220F range for correct lubricity.The inside temp gauge of all Nissans is heavily damped [compresses the reading around the midpoint] to read normal till a real overheat begins [then there is a delay built in].

Check the temperature with the Consult diagnostic computer to see what the real steady state cruise temp is!

Remember if the thermostat is to cold or not in at all the ecm thinks the engine is still cold and stays in open loop longer or won't shift to closed loop and the engine will run richer all the time.

Adding extra fuel washes the oil from the rings faster and increases ring wear, why besides all the other downsides you want the engine in closed loop as soon as possible. At 195F the ecu starts removing [in 1 degree increments per 5F above 194F] ignition advance to protect the engine from what it thinks is the beginning of an overheat.

One of the reasons [besides the normal loss of air density in hot weather] that owners seem to feel a loss of power in Summer. By a 220F coolant temp the advance is down by 5 degrees vs winter.The knock sensitivity increases as the head temperature increases [from coolant not being able to lower it as the differential temp [cylinder vs coolant narrows].So the Knock counts increase to the point where an extra 5-10 degrees is removed on top of what the coolant rise takes out!

A summer dud!