If it is locked up, do the right thing and just remove it.
What will you gain by possibly breaking something or causing more internal damages just to get it to possibly move.
If it is locked up it needs a rebuild. No magic in a bottle is going to make it run correctly.
If you insist on this , more leverage turning the flywheel/convertor with the proper sized tooth wrench. but it not not end well.
If he doesn't know it is "locked up" why did he say it was?? Have to assume people know what "locked up" means. But hey maybe it really means "ROTATES VERY SLOWWWWWW.If he hasn't put a bar on it how does he know it's locked up? Could be a bad starter.... Could be a locked up water pump & the belt prevents the crank turning.... In my world I diagnose before pulling an engine..
It will also dissolve the pistons. Just so everyone knows this beforehand.Vinegar will dissolve the rust sticking the rings to the bores
As was said if it's "locked" or even if you get it to turn in the vehicle, you for sure will want to open it up BEFORE you try to fire it up. I also say if ya want to try and save the engine PULL IT OUT, no short cuts, as they can get expensive!!!It will also dissolve the pistons. Just so everyone knows this beforehand.