The O.S. #8 is a Medium Hot plug for 2-strokes. The F-type plug is a hot plug for 4-strokes, but it will run fine with most engines that will normally use the #A3 Hot plug for 2-strokes. The F-type plug is fairly pricy because it has a thicker coil than a typical plug. It's designed to hold heat longer, which is ideal for 4-strokes.

Why is a thicker coil more expensive? Most glow plug coils contain a bit of platinum, and even tiny amounts are costly. After a while, even F-type plugs will lose enough catalytic materials from their coil that they are no longer efficient.

The next time you're out at the field and you're enjoying some aerobatics, try not to think about all of the platinum that you're vaporizing!