as glow plugs get older, their temp does not decrease or increase unless there is something such as oil caking to cool it off or insulate it which will also begin to inhibit combustion and you will notice it very easily. about the glow plug tempuratures, you want the one that works best with the engine and fuel you are using. higher nitro means it is more explosive and requires a cooler plug to prevent pre-ignition. if you go too hot on a hot plug, you will notice a serious loss of power, and an awful knocking sound. keep it up much longer and your engine is ruined. a plug that is too cool will also reduce power a little bit, but not by anything significant. however, your engine won't idle as well because your plug is becoming to cool to maintain the necesary heat to ignite the fuel properly. basically, tell me if you have a two stroke or a four stroke, and what percentage of nitro you are using and i will tell you your best bet. that doesn't guarantee anything, but i can give you a suggestion. the difference between a two stroke and a four stroke plug are as such. a two stroke fires every revolution so your glow plug doesn't have to be as hot. but on a four stroke, you are only firing once every other revolution on a slower speed anyway, so your plug has to be much hotter. needless to say, your sour stroke is designed to handle that hot of a plug so you don't get pre-ignition and will not run well at all on a two stroke plug.