Coat the prime rib with vegetable oil and Montreal steak spice.
Place the prime rib in a roasting pan uncovered in oven for 30 minutes. After 30 minutes reduce temperature to 275°F (135°C Gas Mark 1) for 1 to 2 hours or until the internal temp reaches 145°F (63°C).
Remove meat from the oven and rest for 30 minutes covered with foil.
Rare meats measure 120°F - 125°F (49°C - 52°C) with a bright red center that grows slightly pinkish towards the exterior.
Medium rare meats measure between 130°F – 135°F (54°C - 57°C) and are characterized by their extremely pink center portion that grows brown towards the exterior.
Medium meats have a light pink center, brown outer portions and readings of about 140°F- 145°F (60°C - 63°C).
Medium well is not pink at all and is achieved at 150°F – 155°F (66°C – 68°C).
Well done is reached at 160°F (71°C) and above and is characterized by a uniform brown color.