It's likely that the soft lead bullets would jump--or rather scoot a little bit at a time--out of the case under the recoil of heavily loaded +P bullets. If they scoot out far enough, you could potentially get reduced accuracy; but more seriously, the bullets could slide out to the point that the cylinder can't revolve, and viola: gun won't work.
Whenever you choose a defense load for your 342, you need to test the bullet--of whatever type--for this phenomenon. Load the cylinder, start shooting: each time you shoot a round, open the cylinder and look for 'creep' from the other bullets. Then put them back in place and keep going. If you can get all the way through a cylinder without noticeable creep from the fifth round, you're doing good. To make even more sure, take the fifth round out and make it the fifth round in the NEXT cylinder--so you'll be able to examine it for creep after 9 shots.
More generally, yes: you can shoot lead bullets, no problem. Standard velocity 158's or Wadcutters should present no problem and shoot just fine in my 340PD.