What type of detergent must I use in the WFW9400SW00?

You must use only HE (High Efficiency) detergent, which will be marked 'HE' or 'High Efficiency' on the package. Using regular non-HE detergent will likely result in washer errors, longer cycle times, reduced rinsing performance, component failures, and noticeable mold or mildew. Follow the detergent manufacturer's instructions to determine the correct amount to use.