Using a wide bench chisel, place the bevel side towards the waste and hit lightly on a perpendicular plane to your workpiece. This makes a definitive guide for your saw and cuts the surface wood across the grain to avoid splintering.
Diagonal chops with your bench chisel can help to relieve some of the pressure in the waste but be careful not to fracture the shoulder lines of your dado with the shaft of your chisel.
Danish oil is a general purpose natural finish for wood. It adds depth and richness to any wood species and protects and seals at the same time.