There's no such animal as a downcut ball unless you get one custom ground...even so, the geometry of the bit will not cure the problem.

Small bits NEED high RPM to shear off the material cleanly. My suggestion is to crank the RPM all the way up (assuming 18k...faster is better) and keep your feed rate somewhere in the 1 to 1.5 IPS range.

Some materials are just stringy or hairy from time to time - oak being no exception. I've carved my fair share of stringy oak...Even boards made from the same tree can show a condition where one machines beautifully and the next is a stringy mess. It happens. We all want 'finish ready' work coming off the machine, but sooner or later you have to break out the sandpaper - or maroon scotch pad (for 3D). Such is life working with natural materials.

Be sure to 3D raster parallel to the grain. Going across the grain will amplify the problem, as will offset.

-B