I would suggest your mechanic is fibbing.

Higher octane fuel does not burn faster or slower than lower octance fuel. The octane rating is simply a number that shows how reistant the gas is to burning upon compression. A "93 octane" fuel has the same properties as far as resisting detonation as a mixture of 93% octane and 7% heptane. Octane is much more resistant to detonation and therefore a higher octane number means higher detonation resistance. And why would how fast the fuel burns leave residue on the injectors? This mechanic does have previous experience with motor vehicles, does he?

Follow the owner's manual and find a new mechanic.

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record