Helical gears - normal gears have the teeth cut straight across the thickness of the gear wheel. Helical gears are cut at an angle so that when they mesh they have a greater surface area in contact. They're more usually associated with high torque gearsets in sniper rifles etc. To be honest, setting them up perfectly so they are silent is a MASSIVE pain in the arse, a well shimmed standard gearset should be almost as quiet and a lot easier to achieve.
Grease - you need a lubricant sufficiently sticky to stay on the gears and not fling off, but with low coefficient of friction. Lithium grease always works well for me or standard Abbey CT2. Silicon is not really robust enough for gears, fine for piston runners though.