I've got two UV flashlights - one at 345nm and one at, I think, 340.
They both work, but the oil seems brighter/more contrasty on the 345nm one.
You also really need it as dark as possible. Even at night, with the lights off in my hangar, I get enough ambient light from outside sources that it diminishes the glow effectiveness. I take a couple of moving blankets and cover the motor and then get under them. It helps a lot.
Without this, I never would have noticed that my fuel pump was leaking. I am now trying a couple of other options before I replace it.