Head gasket

If oil is leaking outside, how would it mix with water? The head gasket could be corroded through by one of the drain back passages. If you have access to a "smoke machine", you might be able to find the source of the leak by forcing the smoke into the dipstick tube. It will only be under 2 pounds of pressure. That might not be enough to make the leak show up.

You can also wash the area, then add a bottle of dye to the oil. Drive it long enough to cause the leakage to show up, then use a black light to locate the source. The dye will appear bright yellow.

Caradiodoc