[sammyg2]

There are many products out there thet get used as penetrating oils. Some are good, others are junk.
The best I've ever used is called Kroil or aero-Kroil.
PB-blaster is good, Liquid wrench isn't bad. WD-40 and other similar produsts are for door hinges on the closet, not cars. They are not penetrating oils. They will displace water, that's about it. Pure kerosene works better than WD-40 as a penetrant.
Get some real penetrating oil. Spray it on liberally. Wait 2 minutes, hit the nut hard with a hammer 10 times. Spray again, repeat, repeat again.
Let sit over night. Get a big breaker bar like the others suggest and crank on that bad boy. It will come loose. I've broken a 1/2" craftsman breaker bar on one, I took it back and got a new one free. Those buggers can really be tight sometimes.
BTW Harbor Freight tools sells breaker bars for less that 1/4 of what Sears charges and are still guaranteed. If you break they replace no questions asked.

MAPP gets much hotter than propane, not as hot as oxy-accetalyne (sp?) but hot. If you have to you can heat it up with MAPP gas and that will make it easier to get loose.