Guys piston slap as it has been called here can come from several things,,not willing to tackle them all in this post but will use adlumia's Honda as an example of one problem. The pistons all heat up from thermal expansion,some expand more than others depending on how they are designed. His motor suffers apparent wear in both the cylinder liner and the " cam" of the piston so now it takes longer for the knock to go away because of wear,the piston still expands but cannot expand to compensate for the wear . Moly will not cure piston knock. What it will do if plated will reduce but not eliminate further wear while knocking when cold,I do not believe that Moly plates to soft metals like pistons,the motor is going south slowly,,you can slow it down with moly plated to the cylinder walls but a higher VI oil will help more but theres a catch.These cylinder walls are lubed from the spray of oil coming out of the rod sides and probable some splash from the crank throwing oil around,,if the rod side clearance is not adequate you will get less oil spray when cold if you get the oil too thick.
I think if the cylinder walls and pistons are scored a bit" what I like about a boroscope " the Schaeffers # 132 would be a means of making a bad situation better for less money with a Dino oil like this that would not be as likely to become overbased
Additives
Molybdenum 1
Sodium 0
Magnesium 13
Zinc 1030
Potassium 1
Phosphorus 869
Calcium 1900
TBN 9
VIS @ 100c 12.2
or possible other oils to include Syn lubes that Molakule would be better in advising to use with the 132 Schaeffers Moly Additive than I.There is more to this 132 than just Moly as discussed previously.
Some of these motors will knock when cold and give long service after they start this but moly alone will not stop the knocking sound,,will just reduce wear from this problem.
When you see a Briggs and Stratton owners manual that says not to use a certain grade oil below a certain temperature becasue it will increase wear or damage the piston this is the splash I am talking about,,just could not insert above.
Ask Bob what the 132 Moly does for oil burners he has seen. It will help fill in the wear scuffs and stablize the rings. If it is doing that it will cushion the blow when cold without getting the VI too high in proper amounts added to the oil.