[Gelbster]

The Pelican rings are HNBR.
Thickness mystery solved.The annular grove for the coolant seal is 3mm. For the oil is 2mm. So 1 mm of compression to seal. Hence the importance of the thickness dimension I mentioned earlier.The Parts Container oil "o" rings are too thin !Yes, they are thinner than the specified 3mm. The Pelican part is a full 3mm.
Just like JFP said , the OEM seals worked perfectly. The ones from Parts Container leaked immediately. The issue was below spec thickness of the oil "O" rings from Parts Container. As I mentioned ,these came as part of a large engine rebuild kit that I used. Now I am wondering about the other seals and parts. The Heat Exchanger seals was a relatively easy fix. Other parts would be much more difficult and potentially destroy a newly rebuilt engine.
To be clear, the issue in this specific case was not HNBR/Buna/Viton. That is only because the engine has zero miles. The oil seals leaked when the engine started for the first time after the rebuild.And yes, of course the 4 bolt were torqued correctly and the base of the HeatX is flat.
As my measurements above showed, if you have an undersized thickness of the "O" rings and the factory machining allows for a max of 1mm of seal compression, it will leak. I was lucky to catch it before any damage occurred.
I hope this helps others with the same issue or choices in sources for seals.