There are two gaskets that are used at this port on the oil pan above the oil filter.
The one that enoniam mentioned has two round holes in it and is used when the truck has the two oil cooler lines running to the oil cooler mounted inside the driver's side tank of the radiator.
This gasket sits between the oil pan ports and the aux oil cooler line adapter.
The other is used only with the block off adapter and it has a single oval shaped hole since the oil simply moves between the two ports on the oil pan through the block off adapter.
I certainly would not use the gasket with the single oval hole when you are using the aux oil cooler or vise versa.
When I bought a new oil pan a few months ago, it came pre-installed with the oil block off adapter and the oval hole gasket.
My truck has the oil cooler, so we installed the two hole gasket and tossed the oval gasket and block off adapter.
I guess I am surprised that I have never seen anyone mod their engine to have an aux oil cooler.
All it would take would be a radiator with the oil cooler ports built into the driver's side tank of the radiator (which it is possible that many trucks out there have a radiator with those oil cooler ports at that location, but they are just not being used.), a oil cooler line set and the gasket with the two holes instead of the single oval hole.
Easy peasy.
A word of caution, the two bolts that hold either the block off adapter or the oil cooler line adapter are torqued low at something like 18 INCH POUNDS.
Someone before I bought my truck had apparently over torqued those bolts, probably to address an oil leak.
I had been fighting an oil leak at that location for the past six years.
Many attempts were made to secure that oil leak and we finally reached the point where there was not enough good material left at those bolt holes to get a good grip on the mounting bolts.
My last attempt to solve this issue was to buy and have installed a brand new OEM oil pan.
The oil leak at that location has finally been solved.
Other oil leaks were still there, but those are stories for another day and another thread.