Use the thinner oil on newer engines.
Most modern engines run 10-30, the lighter oil reduces friction and oil slosh resistance giving better mileage and more engine response and power.
Stay with a thicker oil on worn motors as you need a layer of oil to keep parts separated and take up wear that a thinner oil cannot.
You may even notice a drop in oil pressure due to it being too thin and just running out of the excess clearances.
I use Nulon Synthetic (semi) 10-30 low friction oil on engines, but modern ones.
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