What type of detergent must I use in the WFC7500VW washing machine?

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 create too much sudsing and will likely result in washer errors, longer cycle times, and reduced rinsing performance. It may also result in component failures and noticeable mold or mildew. (Page 12)