I'll just steal a few quotes from the above link to demonstrate the kookiness of this:

**"O2 permeates faster through rubber than does N2...**

Since that is correct, your tires would approach 100% Nitrogen over time.

If you start with 79% Nitrogen and 21% Oxygen, and you are losing Oxygen preferentially, your Nitrogen concentration increases. Even after adding with 79% N2 and 21% O2, your N2 concentration is higher than air.

With each fill you will have a higher and higher nitrogen content. Just one more reason to be suspicious of all the "benefits" promised by Nitrogen fill proponents.
If you do the math, and assume that only the oxygen seeps out of the tire, and the nitrogen remains (not totally accurate, but let's go with that assumption for now, and ignore water vapor):

Initial tire fill-up: 79% N2, 21% O2

1st top up: 96% N2, 4% O2

2nd top up: 99% N2, 1% O2

3rd top up: ... 99.9% N2, 0.1% O2

So in a few months after owning the car, you have essentially 100% nitrogen in the tires, for no cost.