The April/May 2007 edition of Machinist's Workshop did a test of penetrating oils where they measured the force required to loosen rusty test devices. Buy the issue if you want to see how they did the test. The results reported were interesting. The lower the number of pounds the better. Mighty interesting results for simple acetone and tranny fluid!
Penetrating oil . Average load .. Price per fluid ounce
None .................. 516 pounds .
WD-40 .............. 238 pounds ... $0.25
PB Blaster ......... 214 pounds .. $0.35
Liquid Wrench ... 127 pounds .. $0.21
Kano Kroil ........ 106 pounds .. $0.75
ATF-Acetone mix.. 53 pounds .. $0.10
The ATF-Acetone mix was a 50/50 mix (1 to 1 ratio).
That test came out a while ago. After it came out I tried the ATF / Acetone mix. It works ok, and is pretty cheap per oz, but not as good as they make it sound. I have gone back to Kroil, Thrust and PB.
In the real world I still find PB to be one of the all around best, especially for the price. I also find that non "scientifically rusted" nuts, studs bolts etc. come free much better with PB or Kroil than the ATF / Acetone mix.... Kroil used to be great but the cans I get today are not what I feel it used to be, don't know why, though it is still pricey..
Last year I was doing the brake rotors on my wife's Honda Pilot. The set screws that hold the rotors on are notorious for rusting in place as are the caliper bolts. After two hours of soaking the right side in ATF / Acetone I had only stripped the Phillips heads of the set screws, even with an impact screw driver. One of them finally broke (Doh'!). I blasted the remaining two with PB and waited about 5 minutes. The one unbroken screw came right out with a quick blast of the impact driver despite having almost no grip left to it. I put my Knipex pliers on the broken one and was able to get it out. On the left side I simply sprayed with PB, waited five minutes, and both set screws backed right out with no damage to the heads..... These of course were not "scientifically rusted" on and had been naturally rusted, by real Maine winters...
Thrust is also an excellent product. Liquid Wrench makes numerous products and I find one of them to do okay but not the others. Can't remember which one I like though..
Another old timers trick is Coca Cola.. I still have my ATF / Acetone mix but find I use it pretty rarely.. I suppose if you have lots of "scientifically rusted" parts then the ATF/Acetone stuff would be good..