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I have bought recently Mazda Premacy 2.0 , imported from Japan. My question is related to the choice of the appropriate engine oil. People from service nearby have said that genuine oil 5-W30 could be replaced with 10-W40 without any negative effects. What is correct?

If the manufacture recommends using a 5-w30 oil I would try and stick to their recommendations. Its unlikely to cause any major issues however the cost difference between these oil grades is relatively low.