Why do I get brush marks with oil-based polyurethane?
My hunch is that you're applying too light a coat and/or working with it too long. I've used Minwax poly many times, and it's almost miraculous how it flows to a smooth finish. It should look very glossy after you brush it on, and you shouldn't work it longer than necessary. Your final lap should be within a minute or two of application.

I tend to apply fairly heavy, then work it out to the point where it doesn't run. Get off it as soon as that's done to give the surface skin time to flatten itself out. Any brush work done once the poly starts to set will remain. 

Note that I never sand beyond 150 or 220 grit, depending on what I have on hand. Polyurethane spans that texture just fine.