I had my non-certified oil cooler sent to Pacific Oil Coolers (POC) for test/ overhaul. Received the oil cooler looked pretty, install the cooler and flew for .5 hrs. Prior to next flight removed cowling to inspect the cooler, looked bit damp thought it was residual oil. I completed my runup for T/O. Performed a 360 to clear traffic and saw a huge oil puddle. Shutdown, to investigate saw the cowling and fuselage dripping with oil. Towed the aircraft back to the hangar, pressurized the oil cooler and discovered the leak was in the finned area, lost 4 qts. Call POC they said it was an experimental A/C with a non-certified oil cooler, not their problem. I explained POC provided a RETURN TO SERVICE statement and insisted they are responsible. NOPE your experimental. FAA HQ agreed, it is an experimental and POC did nothing wrong. RETURN TO SERVICE statement is meaningless. Side note, POC only pressure test oil cooler to 100 psi.