Our Beta 60 has normally run at about 85 to 95 degC, but has recently dropped to 78

What is preferred engine operating temperature?

Should we change the thermostat?

Engine has run 1700 hours since new in 2013

BTW we have an exhaust gas temperature monitor.  It has consistently read about 20 deg C above seawater temperature, from zero deg in the Arctic to the 32 deg warter we are currently in   Today is reads 50 deg  (we ahve an alarm set at 60)