Is there a fitting similar to the Hayden 397 that will work for the radiator engine oil cooler? I'd like to add an external oil cooler to a couple of our trucks but can't seem to find HOW to splice into the factory GM cooler system like I've done for the transmission side.

 

Not too sure it would be a good idea to use a hose barb on an engine oil cooler. That probably wants to be #10 (5/8" more or less) hose. Factory size on the trucks at least was something near about ½", probably 12mm.

The quick-connect thing that the Derale adapter plugs into, screws into the radiator. I'd strongly suggest taking that out, putting in a straight-thread w washer to AN adapter, and using AN line.

If it was me I wouldn't even use a hose barb for a transmission line. Even there, with FAR less pressure (at least continuously) than engine oil, it's already a guaranteed leeeeeeeek in a very short time. That's kinda just a hack job.