Never soak a piston rod assembly in varsol. A friend of mine did with a 1948 buick engines parts. The wrist pins were loose before the soak. The rod was so solidly seized up after the soak even on a proper press and piston jig the pistons were a little distorted getting the wrist pins out.

I just thought I would mention it rather than anyone ever considering using varsol as a soak.

On the pot cooking to release stuck rings the time out of the pot may be important. I believe the rings are being loosened because of the diferential expansion rates of the aluminium and cast iron or steel rings. The more cycles accomplished with the greatest coolling spread the better. Should result in fewer boiling cycles as well.

Around here in our climate rings stick far worse on gas engines sitting than diesels. I believe the tighter clearances of piston to wall fit in manufacture on diesels tends to retain any oil longer. Or the atmospheric moisture cannot penatrate as easily. More surface tension is present on any oil remaining in the area as well perhaps.

Therfore an older well worn engine will stick easier than a tight one. Just because the clearances are greater.