DO NOT mix fuel oil. All this does is act as a cutter stock to the existing bituminous product and compromises the integrity of the asphalt, and can even compromise base layers (the "bones" of the road), not to mention the environmental impact it could have it reached the aggregate below.

However, "cutters" are typically used in the old asphalt cutback products (literally just asphalt cut back with thinner carbon chains aka fuels/oils). These are being phased out in Canada and the US due to environmental concerns, and the competitive pricing and ease of manufacturing due to emerging technologies of emulsified asphalts, where the cutter is typically very small doses of waste caustic or acidic products, and instead of fuel, literally just water.