Did a little penetrating spray search on the internet, while practicing my Google-Fu.
Turns out it's Liquid Wrench. Seemed to work better than either PB Blaster or WD40 (which is terrible for breaking bolts in general, if you ask me). If you can find PB Blaster or Liquid Wrench or WD40, choose either of the two over WD40.
I also found a forum post that quoted a magazine, which showed an interesting result:
Best Penetrating Oil Test - Surprise Winner
For those of you who are TL;DR:
A 1:1 ratio mix of automatic tranny fluid and acetone was shown to make breaking the bolt much easier; only a tenth of the torque was needed to break the bolt.
Also if you're strapped for penetrating oil (say, if you're like my friend and I, and need more penetrating oil but it's 3 am and the local walmart/major-retail-chain-open-24-hrs is sold out for some odd reason), highly carbonated drinks also work. Not nearly as well as WD40, but much better than nothing else. Vinegar also works, although it's kind of a waste.
There's also that new fangled naturally based (soy) lubricants which are apparently just as effective/more effective while being environmentally friendly. The best part is that they don't smell like nerve gas.