Group 3 oils and extended drain intervals??

[roadfrog]

Let's not start a "which oil is better" thread. I just want to know a couple of things......

I can get Motomaster Formula 1 synth 5w30 (Shell oil makes it for Cdn Tire). It's lots cheaper than other oils in the same grade. I'm assuming it's group 3 oil. Would a 15k mile/20k km drain interval be realistic? A lot of members recommend Castrol, but I understand it to ba a Group 3 as well. I have previously used Mobil 1 0w40, and I already know how good it is, but it's hard to find in my area and it's expensive when I do. For those of you in these parts of Canada, what do you use and how long do you go between changes?

Cheers!!