Not sure there is a better way or place to measure
oil temp. If your sensor is in a remote filter housing it will display a bit lower than if measured in the blocks main
oil galley. This is the reason the
oil is 210+ and coolant is 180+ as it depends where or what you are monitoring.
Maybe just adding 2 quarts to the system when installing a cooler is that buffer that gets you over the hump but it does help. The same extra volume makes a T-stat a good investment.