Well the sputtering is most likely related to the oil fouling of that spark plug. Figure out why and how oil is getting onto that cylinder. Maybe it just needs new 'valve seals'. (They don't last FOREVER). The seals get hard and can completely disintegrate and then the oil just runs down the valve guide into the cylinder. Other than that, the intake manifold gasket could be leaking and probably would be sucking that much oil from the lifter valley. Oil leaking into cylinders tends to raise the effective compression ratio in the affected cylinders, and that usually causes detonation, especially under load, like when accelerating, even if an oil fouled plug isn't firing.( Maybe your 'sputtering problem' is DETONATION.) Probably not the head gasket, you would have many other signs. Oil in the coolant, coolant in the oil, loss of coolant, somewhere... not just 'some oil in a cylinder'. Hmm..."backfire and miss". That fouled plug is is loading your exhaust with lots of unburned fuel air mixture and then igniting ...(eventually). BOOM! You'll figure it out.