In the past three months I've taken the head off of two different 420A engines.
The first motor (mine) was bought with 72K miles on it. Since it's a cheap car, I feel I'm safe to make the assumption that it saw nothing other than organic oil and cheap 87 gas. Since I've owned the car, I've been running Mobil 1 but still 87 octane fuel. The head was taken off at 80K miles.
The second motor (my friends) was bought with very low mileage, around 20K. Since then it has only been run with Mobil 1 synthetic oil and 91 octane fuel. The head was taken off at 113K miles.
The first head (80K - organic) had usual signs of scoring on the cam bearings and the cylinder walls. The pistons were caked with nearly 3/8" of carbon.
The second head (113K - synthetic) had barely any signs of scoring on the cam bearings and the cylinder walls were shiny and glazed. The pistons needed no cleaning, there was basically no carbon build up.
This is obviously by no means a scientific test to which I can publish results.
But I think you all get the point. Running good gas and synthetic oil makes a big difference.