Would using premium gas help create a richer mixture?

The fuel pressure regulator could be causing the lean condition, So both mechanics obviously agree the problem is a lean condition. The octane rating of unleaded pump gasoline has no effect on air/fuel ratio, octane measures a fuels resistance to pre-ignition or detonation... not energy or specific gravity. The fuel containing higher percentages of alcohol will compound your lean condition as alcohol fuels require a richer air/fuel mixture to maintain similar combustion characteristics.