No way in hell would I use a 5W-20 oil in a high-performance turbocharged motor. This lighweight oils are for FUEL ECONOMY. You cannot tell me the engines built today are that much tighter than they were 10 years ago. However, emissions and fuel economy stardards are! I'm using 5W-40 Shell Rotella T Synthetic right now. I really don't care if I get 1mpg less than I did before with 5W-30. In the scheme of things, it's nothing.
Most of the UOD's show a 30 weight oil sheering down to around a 20 weight after 4-5k miles in this motor. Lots of people think this is attributed to fuel dilution, however you'll find that 'most' UOD's done on this car don't show fuel dilution being an issue. I'm sure different driving conditions can attribute to fuel dilution, ie: lots of stop and go, short trips, not allowing the car to reach operating temps, etc... I'm not concerned with my driving habits, as 80% of the mileage I put on the car is at 70mph on the highway. The reason why the oils are thinning is that they're running through a turbocharger that is heating up to 1,000 degrees F and spinning at 100,000 RPM at times. Years ago, manufacturers would give you a choice of viscosities in the owner's manuals and let people choose which oil to use depending on temps. Most of them overlapped. In the summer, I see nothing wrong with using a 5W-40 oil. In fact, the bulletin Neuspeed released for the K04 turbo says to use 20W-50 for the summer and 10W-40 in the winter. I don't know if I'd go THAT thick, but doing a 5W-40 in the summer and then back to the 5W-30 in the winter is my plan.
FWIW, my 08.5 says to use 5W-30 and the one online is the same. Maybe some manuals had typos? I can see the possibility of a tech putting 5 quarts of 5W-20 in the car. Regular Mazda3's outnumber MS3's by like 100:1 and I'm sure it's force of habit. If the receipt says 5W-20, treat it as such. I wouldn't take their word for it. They're just looking to make a quick buck and they're going to hear it from the manager if they find out they had to do another FREE oil change because someone F'd up.