The DC-10-15 was built to the requirements of Aeromexico and Mexicana. They asked for an aircraft that could operate in hot and high conditions. The -15 is the series -10 fuselage with the GE CF6's that were used on the -30. Only seven dash 15's were built!
Just a little background info. The 15 wasnt called the 20 because NW's 20's that were redesignated 40's were still 20's. Utter confusion to follow. Think about it, the 15 was pretty much half of a 30. with the engines and all.