Hi guys, I DIY all my cars, including my girl's 2008 Mazda3 with the 2.3L motor. I have been running Castrol Syntec 5w-20 through it. It's had 4 changes and is at 17K KM - Don't ask, Mazda requires Canadian Mazdas to follow the severe duty cycle maintenance schedule to be eligible for warranty. This means changes every 4 months or 5K KM, and seeing as how she does 12K KM a year, this is resulting in changes at 3K KM. So my question is - I plan on keeping this car for a while and wanted to run synthetics from the second oil change, which I have been. However, at 3 changes a year and 3-4K KM per change, it seems overkill. However, the car does sit outside in +35C to -30C weather, and sees infrequent use and short driving, along with a lot of idling to warm it up before she gets in. So, based on this usage and my oil cahgne frequency, What oil should I use?