Thermostat is supposed to allow the engine to get warm before water being cooling.
One effect of thermostats stuck open is "making oil". usually occurs with boats that run at low rpm for extended periods (i.e. trolling), but has been known to happen in other cases.
Making oil is when unburned gases pass the rings and end up in the crankcase, where it mixes with the lubricating oil. Eventually, enough builds up to where it dilutes the oil, increasing wear. Usually signalled by a rise in oil level and a milky or foamy appearance to teh crankcase oil.
p.s. You do realize you're posting to a 3 year old thread, right?