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Best oil to use

The owner?s manuals for the Suzuki Swift and Mazda 6 we own state that any engine oil from 5w-30 to 20w-50 can be used in our climate, but the dealers use low viscosity 10w-30 in each car. Is this to obtain better fuel economy figures or are there real world tests that show low viscosity oils cause less engine wear than 15w-40 or 20w-50 oils? Also, the Mazda service adviser recommended that I use PULP instead of E10 as some people had trouble with their Mazdas using E10 even though Mazda states that E10 is suitable. What do you advise?

The most likely explanation for the dealers using the 10w-30 low viscosity oil is that they have a deal with an oil company to use it. It’s more likely to be based on a commercial arrangement than anything else. Use the oil specified in the manuals and you won’t go wrong. And Mazda does approve the use of E10 in your car, so I would be inclined to use it.