It is even more important than you think to replace this thermostat.
Engine wear is accellerated in an engine which is at less than operating temperature. It doesn't seem that this would be true, but it is. This is one of several reasons why you shouldn't accellerate hard before your engine is warm. In that case not only do you have the accellerated wear due to cooler overall engine temperature, but you also have oil that is too viscous to flow properly because it is still cold. By the way the oil temperature doesn't come up as fast as the coolant temperature, so just because the coolant temp guage is telling you're at operating temperature, doesn't mean that the oil is.