Oil gets into the combustion chamber either as vapor/mist from the PCV system through the intake system, or measured amounts past the piston's ring pack, by design, in order to lubricate/protect the cylinder walls, or in the case of worn/faulty valve stem seals (both intake AND exhaust) onto the valve stems and therefore onto the backs of the valves.
THIS is
why both a low volatility oil, and an oil with a detergent additive pack which does NOT promote deposits when burned in the chamber are paramount to keeping the intake valves functionally 'clean' in a DI engine.
It is the combustion by products from the even minute amounts of oil being burned in even a fully properly functioning modern engine which contributes to the causes of deposits.
It is NOT that any specific detergents/solvents in any of these oils are 'scrubbing' the backs of the valves (it is a PREVENTION measure these oils seek, NOT a 'fix' or remedial solution to an already existing deposit condition).