In a small bowl, add the pumpkin pie spice, mustard powder, onion powder, garlic powder, a pinch of salt and a grind or 2 of pepper. Stir and rub on all sides of the pork loin. Let rest at room temperature for up to 2 hours.
Add 2 tablespoons oil to an ovenproof saute pan and, just when the oil begins to swirl, add the pork loin and sear on all sides until golden, about 3 minutes each side. Remove to a plate.
In a small bowl, mix together the brown sugar, remaining 2 tablespoons oil and a pinch of salt. Rub onto the pork on all sides. Then place back into the pan and into the oven. Bake until the pork is cooked through, about 35 minutes for medium. Let the meat rest about 10 minutes before slicing.