I began thinking about this because as I was pulling up a long steep dune on a hot day, I noticed that the engine begin to fade as it heated up. The owners manual says to run a cooler plug for "high speed riding" in hot weather, but my experience has been that the bike runs plenty hot even at low speeds. Now, along with changing to 20W-50 oil for summer, I also change to a cooler plug. If you want to be conservative, you can go half a heat range rather than a whole one. The difference is particularly noticeable on the 300EX!

 

400ex's tend to run hot anyways , i guess its a good idea.