Anyone ever use a product called Motor Purr, manufactured, I think by Johnson Products. It purports to be an all purpose valve cleaner, transmission/power steering /engine seal sealer, and all around additive. It's pretty toxic stuff; you wouldn't want to inhale it.

Toxic indeed! Some of those sealer products actually turn gaskets/seals into goo -- I've heard and read horror stories.

If you want to clean carbon etc from the injectors or top end of the motor, try a fuel additive such as Techron or Redline's equivalent.