"outside of" versus "outside"

Hi,

This question's been on my mind for quite some time: I can't draw a distinct line between "outside" and "outside of", it seems that they are both used in quite the same contexts. For example, in American Corpus, which speaks volumes about differemt English usages, these examples are quoted:

1.1. Outside town, the morning brightens as a curtain of sunlight flows into the valley

1.2. Outside of town he pushed the Ford to sixty and let it ride

2.1 He thinks outside of the box

2.2 A bunch of narrow-minded car distributors didn't know how to think outside the box

To cut the long story short, is there any perceivable difference between the two, or they are completely intercgangeable ?

Thanks !