Even modern production engines like the K series still tend to use conventional iron valve guides because for standard road use, they are usually quite sufficient. They do however require generous lubrication to prevent the possibility of the valves ‘sticking’.
In competition use the valves are usually required to operate at higher speeds than in road cars and the oil which lubricates the guides (along with the rest of the engine) is usually operating at much higher temperatures. At this higher temperature the viscosity of the oil in the valve guides decreases and the lining of oil in the guides becomes ever thinner. Eventually this can lead to a lubrication failure as the valve ‘sticks’ in the guide with potentially disasterous consequences.
For competition use we highly recommend using valve guides made from Colsibro bronze. These guides will operate safely at higher temperatures and with minimal lubrication and are much more suitable for valves operating at high engine speeds.