Can I use regular detergent in the WFW9300VU?

No — you must use only HE (High Efficiency) detergent, identifiable by the 'HE' or 'High Efficiency' marking on the package. Using regular non-HE detergent will create too much sudsing due to the low-water wash system, which can result in washer errors, longer cycle times, reduced rinsing performance, component failures, and noticeable mold or mildew. (Page 11)