I have a three-handle Gerber shower, about 15 years old. Today I replaced the diverter with a brand-new one, and I'm having trouble getting it tight enough: it drips inside the wall. A guy at the local hardware store said I should use a black rubber O-ring instead of the white nylon washer that came with the new diverter. Is he right? Or is my problem just that I haven't tightened it enough? I'm afraid to really lean on the faucet wrench lest I strip the threads. If I put pipe dope on the threads, will I ever be able to remove the diverter? Should I put teflon tape on the threads?

Yes, use white teflon tape, at least two full wraps. I would not use and O ring if the manufacturer supplied other. They have no interest in selling aa product that leaks.

Thanks Ballengerb1: two wraps of teflon tape (with the nylon washer and no O ring) did the trick. I'm grateful for your advice.
profSacks