Likely a seal at the back of the HPOP. Not likely that it needs to be rebuilt. Best thing to do is get up into the valley with the engine running, an inspection mirror, a flashlight, and some paper towels. Then you can pinpoint the source of the leak. O-rings are relatively cheap, and chances are one of them went bad. One of mine did a few years ago spewing a few quarts of oil within a couple miles. It was an HPOP plug o-ring.