Some would say that piston slap is not so much a problem but a common condition related to the way engines are designed today. The design of the piston skirt is the culprit.
I know several people with vehicles that exhibit piston slap to some extent. They are Chevy's, Ford's, Pontiac's, Old's and so on.
While many think it can be controlled by the viscosity and type or brand of oil, there is no reason for this to be true. Most all of those that say it works to use 20W-50 or some other type are basing that on personal experiences and not on any known or accepted facts.
That said, I am sure other will disagree vehemently.