Oil Pressure Regulator Question

[frankmako]

My 2004 has started a small leak around the bolt head of the oil pressure regulator. I have done all the cleaning and checking to make sure that it is leaking there and not somewhere higher up. I know the O-ring sometimes goes bad. What is the part number for the O-ring? What size of hex wrench will I need to take the bolt out? Will I lose any oil when I remove the oil pressure regulator? It is not bad, but I know a leak will not fix itself and will get worse.