I could tell you the grand secret of it all, BAforever, but then I'd have to...
I'll say this much: Donald Nyrop of NWA was steadfast in insisting on JT9D's for his DC-10-20 (later rebranded DC-10-40 for a marketing shell game).
But in the late 80's NWA bought secondhand DC-10-30's from - KLM or SAS I think. After a few weeks of flying -30's the crews were less than eager to go back to -40's.
