Fuel - Leaded/unleaded fuel and the alternatives 

The fuel situation seems to be worsening at a great pace these days. And as is exhibited by the questions that appear on the message board many folk are confused by what they can and can’t do, and what is best to do as far as race engines go. Many have posted (particularly from the States) that nothing need be done - just run on unleaded. They've been doing this for years and never had any problems. I have trouble believing that in the face of what I know and have investigated - so here's what the score really is. 
Years ago this stuff called Tetraethyl lead was originally conceived as an octane enhancer. It was soon discovered, however, that the metallic oxides produced by the combustion process formed a protective coating on the valves and seats. This coating prevented micro welding of the valve seat to the corresponding seat in the head under high temperature operation. 
Micro welding causes removal of material on the softer head seat material, causing subsequent recession of the valve into the head. The debris produced by this process is flung into the airflow, causing premature wear of valve stems and valve guides. In particularly excessive cases to piston rings and bores too. Valve recession means that the valve no longer seats properly, causing poor sealing during the compression stroke. The end result being reduced power output, loss of fuel economy, and ultimately replacement of the cylinder head. Reconditioning of a badly recessed head is often not possible as the seats are so damaged they cannot be reclaimed. Some cheap engineering places will do it, but it leaves the valves so far down in the head that little improvement is made once refitted. 
In the early eighties, the toxicity of lead produced by petrol burning vehicles was seen to be a massive health threat and environmentally damaging. After much hue and cry from the ‘greens’, lead levels in petrol were to be gradually reduced to a proposed ‘safe’ level. A bit stupid really because it is either dangerous or not. The average level of lead in petrol at this point was generally around 3.0GPG (grams per gallon), and provided the protection levels needed for a vast majority of the engines it found its way in to. The environmentalists wanted it to be eliminated altogether of course, but not all old type engines would work on this, so the ‘safe’ level of 0.1GPG was set. A huge drop, and was it going to be enough to provide the required protection. 
This whole 'levels' thing was of some concern. As usual, not enough information was available to the general public. ‘Help lines’ were far and few between and those written pamphlets available were sketchy to say the least. Was 0.1GPG really sufficient? Could you safely run a previously leaded engine (4-star) on unleaded or super unleaded fuel? It has become common knowledge that many petrol stations put unleaded fuel in supposedly leaded tanks. This appears to be legal providing that the fuel meets the relevant octane rating - hardly the same thing!! Not many (if any) petrol stations advertise the lead content of the fuel they supply. So, many of you have probably been running unleaded fuel unknowingly. 
The main differences to the car are a fall in performance and possible detonation (‘pinking’ - heard as a rattling noise on acceleration under load). Little initial damage is caused if the engine has covered a fair amount of mileage on proper 4-star fuel. This builds up a protective barrier that will last over a comparatively small period - the so-called 'lead memory'. Hence the advice from some ‘specialists’ that you can use unleaded fuel in your car providing four-star is used every other tank full. Others say it is not a problem as long as you ‘back the timing off’ (retard it) a couple of degrees. 
After much testing by specialist laboratories and engine manufacturers, it was generally concluded that 0.1GPG was nowhere near enough. But the agents supplying data to governments gilded the lily slightly, showing that 0.1GPG would be OK. Just how much lead is needed is determined by a number of things. The major point being the valve seat temperatures. This can be strongly influenced by speed, load, and perhaps most importantly - fuel mixture. 
Lean mixtures were needed to pass emission tests on later vehicles and give good fuel economy produce exhaust gas temperatures several hundred degrees hotter than a rich mixture. Consequently, rich mixtures allow the seats to run much cooler. Tests were conducted under arduous conditions by the independents, and favourable by the ‘informers’. Running an engine under part load (light throttle) for hours on end achieves nothing. We already know that it is under load that the damage will occur (i.e. heavily laden car or when accelerating uphill in top gear). The cynics found 0.5GPG to be a minimum happy level for good protection. 
Even the briefest investigation into what the fuel companies and more particularly aftermarket additive manufacturers were using to replace Tetraethyl lead was mind-bending. Some of the information supplied read like a who’s who of the chemical table world. Others were much simpler. Many merely had supposedly special oils in them to help ‘upper cylinder lubrication ‘, but the temperatures in the combustion process are far too high for these to work. They would probably simply burn off, having no effect at all. Others used race fuel in a bid to attain the 0.1GPG recommended. 
Then there were those that professed to have certain chemicals that at combustion temperatures would phosphide or nitride the relevant surfaces. Extremely difficult to do as both these processes usually command careful temperature control over a specific time period to achieve correct application. Potassium type concoctions are also used, appearing to offer a reasonable chance of success. In reality though, this kind of chemistry causes fairly heavy deposits to build up on the backs of the exhaust valves - decreasing efficiency and seizing valves in guides in some instances. 
The very best option is to use very special sodium based chemistry, which forms sodium oxides upon combustion. This gives the same physical protection from micro welding as lead oxides. This is definitely the stuff to use if you want to run unleaded fuel in an engine originally built to run on leaded. My own personal and extensive testing of additives has proven Red Line Lead Substitute to be far superior to anything else, allowing even race engines to suffer no seat recession when run on unleaded fuel without seat inserts. And no - I'm not on a retainer. 
I've completely ignored the 'drop in the tank' pellets/briquettes or 'in line' filters/ionisers/de-ionisers as they simply don’t work at all. Fact. 
Possibly the best high-mileage, long-term option for road cars is to convert the cylinder head to run unleaded fuel. Basically special exhaust valve seat inserts are pressed into the head, new seats cut, and new, later specification exhaust valves put in. The later A+ series heads use valves with triple-groove collets that allow the valve to rotate to reduce micro welding further.