Hi everyone, were going to get our oil changed at new place Valvoline. They say they have full synthetic or conventional oil. I think we have always used a synthetic blend. We are thinking of using full synthetic but i read online that could make the engine leak in older cars. Our car is a 1993 grand marquis it only has 33 thousand miles. So i was wondering for the people here who have older cars like ours is it best to use full synthetic or convectional?