In the Duramax, the engine oil cooler (stacked plate type) is inline right after the water pump. So, coolant is continuously passing through the oil cooler whenever the engine is running. It's not like a in-rad oil cooler, which sees coolant flow only when the t-stat(s) are open.

The Duramax/Allison equipped trucks do have an in-rad ATF cooler, which resides on the cool side of the radiator, and just upstream from the water pump.

For most Duramax owners, all this is much ado about nothing....

Jim