The manual and the S&W website (FAQ section) states ammo manufactured under "SAAMI" specs can be used . Most major commercial ammunition is made to SAAMI specs, which includes +P ammo. Since +P ammo is higher pressure, excessive use of +P ammo could cause faster wear.

I shoot several rounds of +P in my Shield, every trip to the range, just to be used to the extra recoil. It is too expensive to shoot very much. Who knows how many hundreds of rounds would need to be fired to cause excessive wear? And some brands of ammo are much more powerful (ft lbs energy) than others. Remember that S&W stands by all their recent new guns with a lifetime warranty.

Bob