Combine the whole tomatoes and their liquid, the olive oil, onion, basil, oregano, salt, pepper and 4 cloves garlic in a large bowl or 2-gallon container and puree with an immersion blender until smooth.
Heat a large saute pan over high heat until it begins to smoke. Reduce the heat to medium, add the grapeseed oil and remaining 4 cloves garlic and quickly saute. Add the diced tomatoes and saute for 1 to 2 minutes. Add the pureed tomato sauce and bring to a simmer. Cook for 15 minutes, remove from the heat. Bring back to a simmer when you are ready to plate.