I got a new 2012 Mazda 3 i Touring AT with Skyactiv. The manual says that there's a breaking in period of 600 miles during which you shouldn't drive at the same speed, either fast or slow. I just got past the 600 mile mark. Now, I interpreted this to mean that I shouldn't use cruise control during that time, and that I should try to vary my speed a bit on the interstate every so often. Can anyone tell me how much this affects engine life? Should I extend these practices past the 600 mark?

 

I've got the 2007, but the theory on engine life is that when you drive slowly is to let the piston rings seal properly to reduce the blow by of exhaust into the oil. This helps the oil last longer, keeps compression higher etc...
As for speed, i'm not quite sure why they say that, but one theory is a specific RPM will allow the internal parts to flex just a bit more or less depending on speed, which can help the piston rings stop at slightly different locations instead of having a hard 'shelf' where they had never been before. I find it hard to believe that the 0.002" of a difference this may make has any impact on the longevity of the engine.

I for one did obey the varying RPM rule (it's not vehicle speed but RPM) - though one time I gunned it for a second. After the 500 mile break in I started revving it a bit more through gear changes and then an occasional redline, back to that statement about the exact stopping point of the piston & rings. After 1k miles I changed the oil & filter, then at 5k, and every 5k after that (minus some testing of how the oil changes over time but that's another story)

I'm not a hard driver by any means, but i've had no problems after 65k miles, not burning any oil, no darkness at the tailpipe. The only issue i've had is something kept it from starting a few weeks back (crank but no start), still not sure what that was. But it's fine now and I expect it'll keep treating me well.

Good luck, and after it's broken in, i'd suggest being harder on it slowly to break it into any hard acceleration & high RPMS. But after a few thousand miles it should be good to go for anything you can throw at it.