I would pull the valve cover off. If there is milky crap under there, you can be pretty sure that you are getting coolant in the oil from a cracked head, cylinder or head gasket or you have . As for the operating temp, I drove GMCs of this vintage in Texas heat when we were still using them in the business. We used 180 thermostats year round. A colder thermostat won't keep them from overheating in the radiator is stopped up. It just takes a little longer for a boil over.