Not likely to work, but, you surely could try it.

Depending on whether you can easily remove the fitting, you could polish the taper on the oil cooler in the hope of getting a better seat. You'd need to use a very fine emery paper. You can also take a look inside the taper on the end of the attachment tube. Again, any abrasions here will result in a leak.

I've successfully cured a leak in the brake lines using this method.

One warning: If the fitting to the cooler has been in place for 20 years, it's a slow and tedius process to remove it without damaging the cooler. If you simply add wrench and apply torque, you'll surely strip the aluminum threads on the cooler and then you'll be in worse hurt than you would otherwise have been with the leak.

You need a lot of patience, some PB Blaster, and a bit of heat on the nut to be successful. The nut must be moved repeately back and forth without too much forcing to prevent wiping out the threads.