grade 5 bolts and tighten until good and tight.. really simple.. o ring seal is how most of the later years ones seal,, early ones had flat gaskets.. the old school ones back in the day anyways... these oil cooler aux filter thingies were a good place for a major oil leak and loss of al the motor oil and engine lock up seizure.. especially o the conversion vans etc.. happeen quite a bit actually... the lines/fittings leak or rupture etc and bye bye al l the motor oil i a short period of time and then lcked up motor... i just had an 89 G20 conversion van wih locked up seized 350 motor in it due to this very thing jut last year that i stripped parted out scrapped,, when the oil all leaked out from the ruptured aux oil cooler system, and the motor locked up the battery had actually somehow blown up.. anothe common occurance with thse vans is the radiator cooler rpturing leaking oil into coolant badly... in the stock alum plastic tank radiators, in thse vans.. also happens quite a bit too..