It wasn't the thickness of the oil, it was the additives. I didn't change viscosity...just went with one for high mileage engines. Motor oil formulated for high mileage engines contains seal softeners to allow valve stem seals to soften enough to prevent oil from slipping past the valve stems into combustion chambers, thereby reducing oil burning / consumption. Whatever special additives it has, did affect the gasket material. With no leaks in the first 90,000 miles until the questionable oil change occurred, I'd say the coincidence was when the leak fixed itself after about the next three oil changes using the original oil again, and has yet to show back up.