My 340PD does not have any errosion and I'd like to keep it that way. S&W said the warning on the barrel, "No Less Than 120 Grain", referrs to .357 only. The CS Rep said it is okay to use lighter .38 ammo. That said what should I use to clean after shooting to ensure the gun is clean, but that the cylinder is not damaged?
Hoppe's elite is ok. Ezzox is also good. Call S&W customer service if your not sure what to use. I have a 327PC with a titanium cylinder and barrel shroud. I have used both of the products I suggested and they were fine. The big product to avoid is Hoppe's #9. Of course as you have already noted do not use anything lighter than 120 grain in .357. That will definetly ruin the cylinder finish.
A search will turn up earlier threads on this matter. One shows photos of a PD or Sc (earlier designation) revolver where the owner used light bullet (110gr, IIRC) magnum ammunition. The degree of erosion is frightening.