If you can’t buy it....MAKE IT!!

It likely is not specific to engine oil. As I mentioned, this is what is called "o ring boss" "O ring boss" is same as AN/ JIC fittings, without the flare. It is used in hydraulic valves, pumps and other components, you typically put an o-ring boss fitting into the pump/ whatever, with a JIC/AN flare on the other end and then go to hose. So these potentially have lots of uses.