I usually use only synthetic in my vehicles, but went conventional in my wife's Nissan recently. It got a hole in the radiator which my wife didn't notice until it started misfiring and she then saw the heat gauge was pegged. It damaged the head gasket to where it's leaking coolant into the oil, but slowly. Thankfully it's not going into the cylinders. I have to add a quart or so of coolant every 500 miles, and it'll show a little foam on the oil, though not excessively so. So I picked up a couple 6 quart boxes of Havoline conventional 10w-40 and changing it every 2-3k miles, as needed with dilution, until I can fix the head gasket.