Sump - Term used interchangably with oil pan. Collection area for returning oil and feed source for the oil pump, factory sumps are typically made of steel or cast aluminum. In heavy duty use, the benefits of the impact resistance and resilient nature  of steel outweigh the weight savings(if any) and rust resistance of the more brittle cast aluminum sumps. In street driving, stones, pebbles, or other impacts can crack and break aluminum pans, whereas steel will deform or dent, and not breach.