I read somewhere that when the oil evaporates, the leftover oil is effectively thicker, e.g., to extrapolate, 5W40 may be closer to 5W50 after the evaporation takes place than before so.

I use Redline Oil 5W40 (fully-synthetic ester) in my 2008 Jetta SE 2.5L with 80k miles, which requires oil of type VW 502 00.

Should I still top it up with 5W40, or should I go with 5W30 for top-ups?  

I mostly live in the south (Austin, TX), and top-ups are only required past 5k miles (I don't change oil too often, since I use fully-synthetic ester).