4B11/4B12 oil in Outlander Sport

in Germany and the rest of Europe they said the equivalent ASX motor can use 5W-30. Is it just CAFE regulations that tie Mitsus hands here? Warranty dictates I MUST use 0W-20 but I want to use 5W-30 if there's a better chance of the engine running smoother and reaching 200-300k. I don't care about the ~0.5 MPG loss.