I came across this presentation by Ford on why they require 1,000–1,200 ppm phosphorus for heavy-duty diesel-engine oils.
Presentation by Ford on why they require 1,000–1,200 ppm phosphorus for heavy-duty diesel-engine oils—ASTM Test-Monitoring Center (PDF file)
Presentation by Ford on why they require 1,000–1,200 ppm phosphorus for heavy-duty diesel-engine oils—ASTM Test-Monitoring Center (PDF file)