Oil Cooler Adapter Oil Leak

[AC Ventura]

Although the silicon might work until it fails, it’s not designed to resist oil under pressure and doesn’t lend itself to quick reassembly. Therefore instead of a fix, a proper engineered solution is needed to enable a long term peace of mind. If I understand this right, somewhere along the thread in the block, an oil galley hole has broken through and oil is escaping along the threads and finally, under the tightened bolt head that, has of course no real seal. In this case, I would carefully drill then ream the first few threads out of the offending tapped hole. Hopefully there will be 3/16-5/16 before you get to the oil hole Then I would machine an O-ring groove in the plain part of the bolt shaft, such that when screwed in the (suitably specified) O-ring will engage with the short reamed bore and form a proper seal between the block and the bolt shaft. Obviously you must remove all swarf and you can prevent it entering the engine, by putting a grub screw in first, the blow it out with an air gun. I truly sympathise with the OP’s predicament. Good luck and don’t forget to remove the screw!
If that’s not possible, I’d consider a longer bolt and use a steel washer, sandwiched between two copper washers, that did up against the outside of the adaptor block and not the counterbore. Copper washers are used on brake unions, so they’d sure resist oil pressure.