Going back to the original post, pre-shave soap and pre-shave oil serve two different purposes.
I use my regular artisan soap as pre-shave soap. I think it is smart to wash my face before shaving to get rid bad bacteria and other things that I don't want getting into the micro cuts when shaving. As well, washing gets oil off my whiskers so they can absorb water and soften.
Pre-shave oil holds in the water that soaked into the whiskers during washing or bathing. The oil also softens the whiskers and provides additional glide during the first pass. It's so hot here in the summer that my showers are tepid. Because of this, my whiskers aren't prepped after the shower. I put pre-shave oil on and do other tasks, like putting on deoderant, while it does its magic. Then, when I shave its close, comfortable and easy.