One thing is additives used like cleaning agents.
Other thing is that good stuff like shell is supposed to contain higher percentage of light hydrocarbons which evaporate faster and more easily making fuel to ignite and burn faster improving efficiency reducing emissions and carbon buildup in engine. Also helps in cold starts.
I have done some testing back around 2001-2002 when I was living in Denmark comparing regular stuff to Shell optimax and did feel increase in low and midrange torque and fuel consumption went down a bit making the costs around same in the end. I have only used Shell optimax and now v-power since.
I have done a fair deal of blown head gasket jobs for friends and a couple on my cars some bought with blown head gaskets to fix and sell for a profit. Normally You get quite a bit of carbon build-up in the compression chamber on the head and piston. I have taken apart also my S8 engine due to loss of compression in one cylinder ( it turned out I did not have to take head off because it was down to 2 damaged compensating pieces for valve clearance adjustment causing 2 of the valves not to shut properly but that's another story). I do not know what fuel have been used in it before me but I have done around 8000 miles using shell v-power before taking it apart and there was no carbon build-up at all. Piston tops and compression chamber were only brownish colour with out any build-up. There was some in exhaust ports and manifolds like You normally get. This have increased my confidence in shell fuel.
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