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I'm pretty sure the manual will call for 5W20, not 5W40, unless (maybe) your car is a grey market diesel imported from some market where Mazda sells the 3 with the 1.6L diesel. This is unlikely and seems like something you would quite aware of if it were the case.
As for what oil the manufacturer calls for vs what CTC mechanics will put in, years of behind the counter experience tells me whatever is in the bulk tank is the correct grade.
Anyhoo, 5W30 won't do any harm, it just doesn't get quite as thin as 5W20 when the oil is up to operating temp. In theory this could result in a 1-2% increase in fuel consumption. Personally, I feel this unlikely possibility is a worthwhile trade for increased engine protection and have switched to 5W30 myself.
As for what oil the manufacturer calls for vs what CTC mechanics will put in, years of behind the counter experience tells me whatever is in the bulk tank is the correct grade.
Anyhoo, 5W30 won't do any harm, it just doesn't get quite as thin as 5W20 when the oil is up to operating temp. In theory this could result in a 1-2% increase in fuel consumption. Personally, I feel this unlikely possibility is a worthwhile trade for increased engine protection and have switched to 5W30 myself.