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My Dart was making noise and just replaced the seal (ends up was the motor bearing not the seal causing noise) but no grease is required.

If anything they state, don't touch the seal, just wet the surfaces with water and not use silicone lubricants or grease.

The portion of the seal that inserts into the pump casing that where you use the sealant. I just used some GE Silicone type I, being careful not to get on the face.

Check their troubleshooting page on their web-site, they mentioned something like can tighten up if sitting and to try to rotate using a screw driver on the end of the motor shaft to free things up.