You can get the oversized oil cooler adapter O rings from several of Buick vendors......much better than the original factory O rings which leak in short order due to their smaller crossectional area.......the factory units simply don't compress enough and are prone to leaking.
You can see the difference in this photo....one on the left is original sized factory GM O ring.
Buy a few of the oversized ones offered by vendors like Kirban and several others and you'll be much happier.......they're cheap at around $4.00 each
FWIW, I have NEVER had an O ring actually seal that adapter - no matter what size. I ended up pulling the gasket off of an oil filter and putting that in place of the O ring.
Old post but I took the gasket off an oil filter (pf47 I think) and so far no leaks. Been about 2000 miles. Torqued to factory spec. Thanks John for the tip.