On NGK plugs, lower the number, hotter the plug. A hotter plug, as long as it doesn't cause a hot spot which leads to pre-ignition detonation, will foul less than a corresponding cooler plug.
The BR8ES and BR8EG are completely interchangeable. The only difference is in the electrode materials. Here is what I pulled on them...
S series plugs: These are standard plugs with copper cored centre electrodes. A copper core offers superior heat conductivity hence heat is removed from the firing end of the plug and 'hot spots' are prevented - these can lead to pre-ignition. A copper core also allows a longer insulator nose to be used - this protects against fouling.
G series plugs: These feature a smaller diameter centre electrode tip made of nickel alloy. The smaller diameter means a lower voltage is required to produce a spark. As the tip is made of conventional Nickel Alloy the service life is reduced. Best suited to applications where plugs are changed regularly.
That said, I have only used the S version plugs in all my bikes. That's Honda, Yamaha, Suzuki, Kawasaki, and GasGas bikes and I have never had a problem. That includes my old '74 Yamaha RD350 race bike I had many years ago. I tend to replace the plug once a year whether it needs it or not. Good jetting and a properly maintained engine are keys to good plug life.
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