Synthetics tend be a thicker in viscosity versus a traditonal mineral based oil with the same rating. Because of this, it exerts more pressure against seals and rings and helps seal them shut.

You'll find a lot of times people who begin to burn a lot of oil will switch to a thicker oil grade to slow down burning for this same reason.

Higher octane gasoline tends to gain more detergents to PREVENT carbon deposits but is not going to do a very good job at cleaning thousands of miles of exisiting buildup.

For cleaning up engine parts such as valvetrain components, degreaser and a rag usually works fine. Piston tops usually are stubborn, I've cleaned pistons in the past by using degreasers and a wire brush. You need to be careful that you only use the brush to break up the carbon deposits and not to scratch the fuck out of the top of the piston, the metal is pretty soft.

There's other methods. I mean if you wanted the entire block cleaned, you send it through a jet cleaner or a hot tank.

Synthetics offer detergents that help break down deposits but won't remove them all.