But if it is

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Sorry, Glenn, I don't agree. We are right in the heart of the rust belt, and oil is actually the only truly effective rust prevention we've found. Yes, it does soften rubber, including rubber based undercoating. As far as used motor oil is concerned, I think it would be too thin. I use Fluid Film personally on my vehicles.

James Kuhns
Owner/Technician
Auto Tech Solutions
Chatham, New York, USA