There is a gasket behind the metal piece (oil cooler assy) where the oil filter goes on.
The gasket keeps oil and coolant from leaking. Its about a $15 gasket but you will also have to replace coolant and oil.
I just had to replace that same gasket last month on my Mach. Not hard but just kind of a pain since it gets messy.
The gasket Im talking in the pic is number 6840. Of course that number depends on your year,motor. Please dont just get that part number,I only used it for reference.