I replaced my oil cooler assembly today. The leak took a substatial turn for the worse around 3 months ago and starting dripping onto the ground. Past year it was just making the area below it wet, but it got so bad everything under the car started to get covered. Car has 205k. Job took around 6 hours, including multiple breaks and a lot of time just spending evaluating how to do it without breaking anything. After removing the oil filter cap, I was able to pull it out behind the engine. I did that so I didn't have to remove alternator or DEF injection nozzle. I did have the oil cooler outflow pipe (T shaped one) removed as well since I also replaced that (did the inlet pipe back in January). Having that out of the way helped a bit.
Torque specs:
Oil Pressure Sensor - 26ftlbs
3 bolts oil cooler assembly to block - 37ftlbs
I ended up buying the whole oil cooler assembly brand new (part# 55578737). The seals on the new assembly are the skinny ones just like the ones that I bought in a 2 pack from eBay (mentioned above), and not the thicker style ones that come from the car when new. They may have changed the seals to a different design to fix some of the leaking issues, or maybe the seal compress and stay that shape after many years of use and they are in fact the same? I don't know.
So yes, if you have the oil cooler leak and want to change just the 2 seals on the oil side, you can buy AJUSA set 77001000 (GM# 55485373) from eBay or online and they will work and get out of it with just some labor and cheap seals.
These are the same style seals below #77001000 that came on the cooler brand new.