Add one pint of old original PB BLASTER to the oil and running it a few hundred miles will also help loosen stuck rings. Garage where I used to work did that on mid 60's little old lady Oldsmobiles and Pontiacs that had problems with slow cranking caused by carbon getting behind the rings. By time for the next oil change, the problem was usually gone.

EDIT: Just add a pint instead of a quart.