Smoothing clay

[Supafly345]

I had a set of flat tools of varying widths that I got from michael's to do this very thing. Getting a cheap set of sculpting tools may not solve the problem if you have an entire sculpture of brush damage, but it helps a ton smoothing out any of the detailed areas or corners.
I don't know much about brush maintenance meant for mineral oil, but it sounds like you might just need to buy a more finely bristled brush.

The ideal solution would just to learn how to work faster.