Just thought I would add my 2 cents on this topic as I have gone through this same issue. I have a 2wd 2nd gen blazer. My oil filter adapter block was leaking so I ran down to the parts store and bought the Dorman kit like everyone else. There are two O ring gaskets that appear will work. I picked one and it seemed to work for a few weeks until the leak showed up again.

I recalled having the other O ring and thought maybe I picked the wrong one so, I again replaced the gasket with this O ring. Again the leak returns after a couple weeks. This time I call the dealer and get the OEM gasket. Not much difference in cost of the Dorman kit. Upon visual inspection of the OEM gasket to the two Dorman gaskets I can see the OEM gasket is much thicker in diameter.

So, my advice on this part is get the OEM for this part. Also, the dealer was the only one that I could find that had the oil cooling ling gasket as well.