By John Alexander, of Glass Accessories International, a long-time student of glass-cutting technology and the man behind the ubiquitous Toyo Supercutter:
The outside surfaces of a piece of glass are in "tension", holding the middle in compression. When you score glass, the wheel penetrates less than 1/1000 - inch into the tension layer but drives a median crack well into the compression layer. It's very important that you minimize excess surface fractures as your wheel rolls across the glass. Cutting oil, literally "oils" the score line, reducing this surface damage. It also lubricates the track made by the wheel, reducing the tendency of the score to repair or "heal" itself. It may be messy, but oil improves your score and thus the break-out.
The "hone angle" is the angle to which the wheel is ground. The hone angle depends on the ability of the wheel to have and hold a sharp point, which is a factor of the material the wheel is composed of. A steel wheel is very difficult to get sharp and more difficult still to keep sharp. Steeper hone angles compensate for a steel wheel's inability to get and stay as sharp as more modern alloys, like tungsten carbide. Steel wheels for stained glass had 114 degree to 120 degree hone angles. Toyo's tungsten carbide wheel uses a wide (135 degree) hone angle with a very sharp point.
The difference in impact between a steel wheel (steeper angle-blunt point) and a tungsten carbide wheel (shallow angle-sharp point) has been likened to the difference between the effect of a 250 LB. man in wingtips and his 98 LB. companion in high heels, both walking on a hardwood floor. The pressure applied to the glass is directly proportional to the amount of wheel touching the surface. This is one of the reasons carbide wheels have up to 25 times the life of steel wheels.