New engine use 0w-30 or thin oil , but after period of time, engines start wearing down(seals dries, cracks, compression drop ...) so its better to use thicker oils.Oil will start burning or leak...
Also different oil brand make change in fuel economy even if they have same viscosity. Weather, driving habit, traffic car condition and fuel also change fuel economy. its not just the oil. But it make a difference.
If you want to use thin oil, try to use 0w-30 or 0w-40 5-30. Thicker oil in summer , thinner in winter.
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