Kelly, since you're new and may not be acquainted with the acronyms, MMO is Marvel Mystery Oil. In that engine you could probably just throw it in on top of the full sump without a problem. I don't think windage is an issue in the GM V6's.
You can also add it to the fuel, but in this scenario you're only concerned with the oil. It's a solvent that might clean up some deposits if you have a fouled lifter.
In my experience piston slap was more pronounced under load. Almost like a muted rod knock until warmed. While a lifter tap would be boldly apparent at idle and throughout the rpm range until it might be drowned out by other noises.
..but that's just my experience.
Now you may have a condition that mimics piston slap in it's onset and duration and it can have an origin in the fuel. It could also explain your rough running until warm up. It's called CCDI (combustion chamber deposit interference). It's from shorter trip usage and fuel residuals creating deposits that are relatively soft and bang around when they reach a certain level of accumulation.
Get a bottle of Amsoil PI, Gumout Regane COMPLETE, Techron Concentrate (all of them around $10+/-) and follow the directions in your fuel. This will eliminate that as one possible origin and is wise preventive maintenance on a 6month basis anyway. That is, it won't be wasted money even if it doesn't solve your problem..
Good luck.
