Valve seal stops oil from going down the valve and inside the c. chamber.
That is the job of the valve seal. So ask your mechanic how is this connected to fuel consumption. Another good question is whether oil or unburned fuel is fouling your sparkplugs. May nag adjust ba ng AF mixture or iginition timing.

There are three ways to trouble shoot a problem 1)Systematic Diagnosing 2) Elimination Process 3) Hit or Miss Method. It looks like he is doing the last.

Doon sa last tune up mo, ginalaw ba ang air fuel mixture mo? Timing? Ignition?
Remember gasoline has oil. Baka kasi over delivery, delayed or advance.