Did a bit of research on glow plugs before trying different ones, changing glow plugs is another way you can use to tune your engine, one thing you have to watch for is detonation (advancing the timing) with a hotter plug, I don't think a cooler plug will damage the engine.
Here are the big lines from the OS site
Guideline : Know what type of engine you have. Is it a standard - or a turbo
Guideline : The smaller the engine, the hotter the plug.
Guideline : The higher the nitro content, the colder the plug.
Guideline : The hotter the day, the colder the plug.