Oil pressure regulator? Not familiar with that part. Sending unit or relief valve maybe?

Anyways, on the leak. Not saying this is the problem but others have experienced this on the 3.5 DOHC, so its worth a look. I had a leak like that one time right after a timing belt change. Turned out to be oil dripping out from behind one of the bolts that mount the hydraulic tensioner to the engine. These bolts pass through the oil pump and into the block. My theory is the gasket back there between the oil pump and block goes bad. Anyways I fixed it by putting a small amount of RTV on the back of the tensioner bolt hole and a copper sealing washer behind the bolt head. Also put teflon tape on the bolt threads (not sure that step did anything as threads are in the block) May or may not be your problem but worth a look.