The CSK noise is something you are going to have to live with. I know of several guys with GM trucks with over 100,000 miles and their engines have never broken down. One guy has a 1999 and it made that noise within the first 10,000 miles and still to this day. Does not use oil and running strong. He sells insurance and logs over 100+ miles daily. He told me its the best truck he ever had and his company has had Ford and Dodge. They buy all brands because they have dealers as customers.
I think there are 2 things going onl some engine have piston slap as described and some engines just make noise due to disimilar metals in the engine which expand at different rates. I have heard the noise and if its pistons, why do you hear a knock noise about 1-3 times a second when the piston is moving some 70 time per seconds? I had a 1990 V6 Lumina, a 1993 Lumina, 1999 Monte Carlo Z24, a 2001 GMC and each one has made a noise when cold that disappears when the enigne warms up.
As for the noise being helped or eliminated by a particular brand or weight of oil. That is wishfull thinking. The service manager of a local GM dealership whom I have known well for over 20 yrs told me they have tried all the different oils etc and will some seem to imorove it, it only last a short time and the noise is back. He also said that he has seen trucks exhibit the noise and then not make a sound for miles and miles, then start at it again. There are some theroies that the noise is the result of carbon forming on the pistions, when cold a noise is made by the interaction with the engine firing and the carbon.
I know some strongly disagree with this and claim it harming the engine yet the amount of engines failing from this is small. Actually those engines that have been rebuilt or replaced have the same noise.
This is my opinion, right, wrong or indifferent.
I own 2 of these GM engines, 2001 5.3L and 2002 5.7L LS1