New York Salt
Having just traded my ‘16 Tacoma in on a new ORP, this thread is a little disheartening. The truck was recalled for inspection of the frame, where they applied generous quantities of seam sealer in various areas of the frame - right over the un-prepped rust. Pretty sure it did nothing. Anyway, they deemed the frame acceptable and sent me on my way. No pics to share unfortunately, but it looked far worse than the OP’s 4Runner.
Around me, I think it’s a calcium chloride brine they apply on the major roads, in addition to granular salt. Don’t know if that’s better or worse for our frames.