Unreal supports specular as some kind of legacy thing right? But sometimes it's useful. But materials in Substance are either spec/gloss or metal/roughness. Is there a way to preview a material in Substance to mimic what the extra specular tweaks in Unreal can do? Or is that pointless and I should just forget about…
If you add a specular channel to a metal/roughness project in Painter or Designer, it will be used by the default shader the same way as Unreal uses it, to modulate the overall strenght of the specular term.