duramax oil cooler change
oil cooler is connected to coolant system, so you will lose some when you break it loose. Oil too of course.(I don't know how much)
There is no gasket at the block face, it is 2 o-rings, one on the face, one on the boss that fits in the 28mm hole. Save them and check for damage.
Also check block face surface for roughness where o-ring mounts on face.
Any rough surface should be smoothed over to get good seal.
I manage tools and programs on the CNC machining line that cuts this area of the block. There have been some leaks reported due to poor finish on this face where o-ring fits. In 2002 I changed the machining process to improve surface spec. on this face at the request of Mfg.Engineering.
No reports of leaks since then.(none that got all the way back to me)