http://www.network54.com/Forum/thread?forumid=15639&messageid=1094602108&lp=1094637256
Scroll down about two thirds of the threads where I ask the very same question.
I have done a little research since and I have come to the same conclusion.
In summary, they are pure hype and don't offer any mechanical advantages. They have been manufactured to give an immediate apparent result to convince you that it does indeed offer benefits. When first added, its noise suppressors instantly convince you that your engine is running more sweetly.
If these miracle products did indeed have remarkable anti-wear characteristics then surely we would see more advertising about it on serious motorsports, more mentions of the product in specialised mechanical publications and on the packaging of high performance, expensive, fully synthetic oils. (you are about to spend £x's on a high performance alternative, they could very easily make a little more claiming that blah blah this that and the other, you are their captive audience).
To counter that, the miracle oil advertisements talk about customer satisfaction and a 100% refund. This automatically instils confidence in a product you haven't even used yet so you ignore the general disregard and scepticism.
As the other Tom states, engine oil degrades primarily when exposed to extreme pressures and temperatures, he highlights camshafts as the main area where this occurs.
The miracle adverts do not show these additives being subjected to pressure, only extreme cold (which the engine oil viscosity ratings already have covered in its specification) or oil starvation by detatching the sump.
They don't examine the damage in huge detail and they do not revisit an engine after thousands of miles to do a comparison.
Mobil 1 is regarded by the average consumer as a very high quality, fully synthetic engine oil, their main focus is on the stability of the product and it's suitability to most engines. Surely they would have taken the time to stress the point that it too included miracle anti-wear characteristics, had it been a scientific requirement.
Highly specialized racing oils stress temperature stability as the reason to use their product.
My conclusion is that the likes of duralube are simply placebo products.
Shelves are full of additives that are designed to be added just before an MOT to pass the tests, after 100 miles or so its "benefits" have been expended, but they make it clear that it was only designed to help you pass emission tests, leak tests etc.
Ask yourself if you want to deal with the problem or temporarily with the symptom.
