How to loosen a frozen engine.

No oils typically work only if it's barely stuck.  Even ATF and acetone.   
 The problem typically is corrosion where the iron rings first stick to  iron cylinder wall.  Due to the acids the develop in unchanged oil.

      Over time the corrosion expands in the piston ring lands freezing them so even if you could move the crankshaft  you'd have to rebuild the engine anyway.    
  You need to dissolve the corrosion.  The only thing that does that, is acids.  Careful though too strong an acid and you'll start etching the cylinder walls. 
    White vinegar is what you want.   Cooking white vinegar is about 10% acidic.  Look for cleaning white vinegar. That can be up to 30% acid.   I find it at Home Depot but in the household area.   I'm sure other places have it as well.  
        Pull the spark plugs and fill up each cylinder.  A day or two later you're ready to put a long handled wrench. On the crankshaft bolt ( the big one in the center)   And try.   I've never had it fail on me. Although sometimes I'll drain the oil and vinegar from the pan and give it a second soaking.   Once  it pulls over easily, I'll squirt some ATF and acetone in to lubricate the rings.  You only need less than 1/2 cup per cylinder.   At that point I  fill the oil pan hook up a fresh battery and really spin it over.  Now comes about 1/4 cup of motor oil and a good spinning.  Reinstall the cleaned spark plugs and Bob's your uncle.  

It's a sign to swap in a small block. 

Don't bother; the fact that it is stuck means whether it's rebuilt or replaced, it's got to come out.

Funny/relevant story-

An old junkyard/fleet mechanic friend of mine used to handle the local school bus fleet as well as strip incoming junk cars before the crusher.  He had a Saturn SW2 come in completely seized and devoid of any oil in the pan.  Wanting a lot car parts hauler, he showed me this trick.  24hrs and it turned over with a 4' pipe on the crank.  48hrs and it was just the breaker bar, working at it every few hours or so.  

 At 72 hours, he turned it over on battery,  reinstalled plugs, filled it with USED diesel motor oil from one of the buses, and i'll be damned if it didn't fire right up. 

He used that poor wagon for a good 10 years running tools and parts around the yard... He'd just fill it with used oil every time he added gas, which was also just crap that he'd siphon out of incoming junkyard gas tanks, haha.  He never ever spent a dime on that stupid thing, and it'd start up regardless of the questionable crap he'd throw in it, haha.   We called it Recyclopse... 

kb58 said:

Don't bother; the fact that it is stuck means whether it's rebuilt or replaced, it's got to come out.

Not true.   Assuming nothing is broken or bent if it's just rusted rings. And you get them free. You might be able to run the engine even without oil smoking.  
 I first did this to a Ford Model A  and with a little carb work got it running decently, idling smoothly ( for an old Ford Model A).  Barely a little smoke even at full throttle.  
     I've also done this trick to a old Chevy pickup with the straight six. 
    
     

Captdownshift (Forum Supporter) said:

It's a sign to swap in a small block. 

Yeh!  It works on small blocks too.