I know S&W says you CAN use +P in any gun stamped with the model number in the Yoke area and I would not be afraid to use the Speer 135 grain +P "Short Barrel" Gold Dot load if it is stamped. If it isn't (as pre M10's should not be) then you are probably best off using 158 grain standard velocity LSWC bullets. That's the safe answer, but were it my dilemma I'd use the CURRENT Remington FBI load (158 grain LSWCHP +P) because according to my Chronograph tests even though it's labeled +P, it is only loaded to standard velocity specs. Sorry to report that, but after testing them several times, that's what Mr. Chronograph says..........
I do not represent myself as a Metallurgist, but I can't imagine 6 anemic Remington FBI loads causing any harm if fired in anger as ALL S&W guns do have a certain margin of error build into them. As far as I can tell, todays +P rated FBI loads (Big 3 manufacturers) are yesterdays standard velocity loads.