The article above I think gives the true answer by way of a round about manner. But I do disagree that the root cause for engine sludge is the use of high octane fuel with synthetic oils.
In my opinion, I believe that some of the reasons are engine sludge to form are the following
High engine temps
Hard driving
low quality fuel
Bad filtration systems on the oil circuit
Low operational temps
It is true that high octane fuels raise the engine combustion temp. But this alone is not enough to breakdown the oil and cause sludge.
On the flip side, some simple practices can prevent the formation of sludge.
use of a good oil filter - As important as the oil itself, I am a firm believer that my oil is only as good as my filter.
good quality oil - Synthetic is fine
Timely oil changes - If you drive a lot in stop and go traffic, harsh weather, bad air quality, the manufacturers claim of extended oil change periods do not apply to you!!!
There is no point in discrediting synthetic engine oils, as I don't think it is a proven fact that only synthetic oils cause sludge.
On another note, I am not a firm believer that just using synthetic oils lets you change oils at longer intervals. the oil change interval depends on
The oil
The Filter
The type of driving
Type of engine
I think this is a much larger discussion. But my simple point is that just by changing the oil to synthetic oils, you may not be extending your oil change period.