kerosene or minerial spirts outside surface and inside the runners. Scrub with a parts cleaner brush available at auto jobbers. Then wash with water. Then use mag wheel cleaner to clean the aluminum outer surface. Keep the gasket mating surface protected from mag wheel cleaner as it might etch the surface.
If plenum on the car use caution and carefully spray with small amounts of mineral sprits and use a parts clean brush and rags. Clean up is limited if plenum on the car.