Pawosd, You are not really using a lighter overall oil but rather one with a lower rating on the bottom end. The attempt to mimic viscosity is a heat driven chemical function I believe. The oil is designed to thicken with temperature.There is perhaps a chance the synthetic is just more functional with the viscosity additives than the dino oil. In otherwords it is managing to sustain higher levels of viscosity at elevated temperatures or perhaps is just more responsive to manipulation than the dino oil. Oil today is a real engineered product. I am not sure totally of what I just mentioned but is my best guess. What you are experiencing does not have to be your imagination. My guess when 5w=50 is marketed you would probably be better off using it. Perhaps it's out there now. As long as the weight 5 is followed by a w it should be completly safe and actually better for a high milage engine. Chemistry will continue to move right along in a serious competative market. Wonder what comes next?