Pennzoil actually responded:

"Thank you for your e-mail inquiry. Pennzoil Concentrated Engine Flush will not have any adverse effect on engine seals and components. Our engine flush is oil based - not solvent or Kerosene base and is compatible with all engine oils.
Feel free to contact our Technical Services Group at 800-458-4998 if you have any further requests or questions."

Maybe the seals that leak after using this product were only sealing with a build-up of gunk that the flush dissolves?

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1986 560SL
2002 Toyota Camry
1993 Lexus