Idle bars are there to prevent the fresh fuel charge from hitting the plug too quickly and causing it to cool off. This is particuarly noticeable at idle. If your engien won't idle low enough and dies by slowly loosing rpm, you might try an idle bar and see if it helps. Some engines like them, but on some it makes no difference. I've also heard (but never experienced myself) that some engines will run poorly with an idle bar plug at high RPM as the ignition gets delayed a tad.

Another variation in plugs is their length. The OS #8 is listed as a "long" plug but it's actually a "medium", there are several "long" plugs that are longer. K&B long plugs (1L, HP, RC), for example are noticeably longer in reach than the OS #8. A longer plug can sometimes have the effect of increasing the compression a little, which some engines like and some don't. On most, you won't notice a difference.

Btw, a "Short" plug is used in small engines where there isn't enough clearance for a "long" plug. Many engines below a .15 use short plugs.

Plugs are frequently rated as "hot", "medium", or "cold". A hot plug keeps the element hotter and therefore causes ignition earlier than a medium or cold plug. High RPM, high compression, and high nitro are factors that require a colder plug.

If you have a plug that is too hot, the engine will experience pre-ignition, and you might hear a frying egg sound when it's running. If the plug is too cold, the engine won't produce full power, and will idle and transition poorly. Most all sport engines run on moderate nitro are set up for a "medium" plug, and most of the plugs you pick up will be mediums. OS #8 is a medium.

In general, if the engine starts up easily, produces full power with no odd sounds coming from it, idles and transitions well, then your plug is fine. You won't get more power from changing brands of plug in most cases, so run what works. I happen to use the K&B 1L in most of my engines. Mostly because it's less than half the price of an OS #8.