I can't know what's involved with your 340PD, but I recently had someone bring a 340PD to my attention because the cylinder wouldn't easily turn. I checked it and immediately agreed.
I removed the cylinder & yoke from the frame, cleaned & wiped off the yoke and added a couple of drops of oil to the yoke's 2 bearings and the stud, reassembled the gun and found that it spun freely and easily.
The alloy yoke used in the Airlite Ti/Sc guns seems to really benefit from the recommended lubrication, but the cylinder and yoke have to be removed in order to expose the yoke's bearing surfaces and stud. A local armorer or gunsmith ought to be able to show you how it's done.
Be forewarned that trying to do this without some knowledge and experience can easily result in scratching the frame and wallowing the yoke screw, and even worse, it can potentially result in someone over tightening the yoke screw and damaging the alloy frame.
Believe or not, S&W suggests in its armorer manual that revolvers be disassembled and checked by a qualified armorer once a year.
I like the last line in the suggested maintenance program section which states,
"A revolver that is well maintained will be practically trouble free and have a much longer service time than one that is not."
Got an armorer or gunsmith locally? Might be a good place to learn what's involved.