I just got my MSF Natural and so far the oxidization problem hasn't been as much of an issue. I love it, it has a great light texture though very little coverage. I really think that StudioFix powder is notorious for oxidizing, as I've heard many girls complain of it.
I've also noticed that the less product I put on my skin, the less oily it becomes. Primer, concealer, msf and blot to seal the deal. So far this is working for me. I guess I'll save the Studiofix for full coverage days when my skin isn't looking so hot.... SF powder is the only foundation that I've ever tried in my many years of wearing MAC, so I'd be curious to see which formulas oxidize the least.
And yes, I have noticed that wearing a primer DOES help.. alot!!! Thanks Dolls.