Jackson's Art - The smoothest shading results can be most easily achieved by using Zest-it Pencil Blend. It is a solvent that will break down the oil and wax content of pencils,
Zest-it Wax Solvent | Art & Craft Supplies at George Weil
Zest-It Pencil Blend | Ken Bromley Art Supplies
Zest It Oil Paint Dilutant & Brush Cleaner (Eco Solvent) - £5.45 - Pegasus Art
What is Zest-it Pencil Blend and how do you use it?
Pencil Blend | Products Listed - Art Supplies from Crafty Arts UK
Pencil Blend (Citrus Free)it's good for blending coloured pencils