I had a 95 jetta that I bought new. It developed a slow leak. I took absolutely no rust proofing prevention (except undercarriage wash once in a while). Half of the undercarriage parts, were severely rusted. The side that got sprayed and coated by the leak was still pristine.(in 2007 when a deer totaled it). I had to replace the bracket that held the radiator because the rust disintegrated it. I only used conventional oil back then, not sure if synthetic would have protected those parts in the same manner.