30% fuel will ignite sooner so if your head isn't shimmed correctly (with some engines you need to take away some compression) you can get pre-ignition. If you have pre-ignition then your engine will be nearly impossible to tune and run hotter along with blowing out glow plugs. Most .21 engines are shimmed for 25%-30% nitro.

They still use the same lubricants in the 30% nitro fuels if not they use better lubricants which can help your engine last longer. Also keep in mind just because something has only 8% oil content like Odonnel's 30% race blend that doesn't mean your engine wont last as long. Its more about the quality of the lubricants then the amount. In fact more lubricant % can make some engines run worse especially the on-road .21s.