Give us a better description of the glow plug (how the element looks) I'm currently using Mccoy MC8 or 9 and they last for a very long time I can't even remember when was the last time I changed it.
Sometimes I keep my 4tec running with out stopping the engine for over 45mins. always checking the temp not to get over 240 F after running full throttle for a few runs. If I remember well I broke the engine with Byron's 20% after a gallon changed to 30% and now I'm running 33% and never added any shims under the cooling head.
It is important to give your car a good maintanance I open the engine every once in awhile to check the internals... after more than 6gals the pitchs is pretty much gone but there's compression no scratches on piston or sleeve.