I doubt the change in oil viscosity would have a significant effect on coolant temperatures. I would run the factory recommends. You will find very few people if any that have seen the coolant temp light come on and many run in similar or worse conditions than what you describe.
Your problem is most likely with the cooling system, not the oil. As suggested, check fans, check cold coolant level, bleed the system and possibly replace the radiator cap although a defective cap will usually result in coolant being purged through the overflow. If your level is low, that may have happened.