Whoa, now! You need to be careful here. The EPA has mandated change in the formulation of 15w40 "diesel oil" within the last 3 or 4 years. It formerly had a ZDDP "zinc package" for very strenuous applications. That has now been removed.Dixie has changed the recommended oil so many times that I'm convinced that any decent oil will work. First I think they called for 15w40, then they called for their own VTC proprietary oil, and finally Lucas 15w40 diesel oil. I'm currently using NAPA 15w40 diesel oil
The EPA is only worried about street use, so you can still get the ZDDP in "after market racing oils" like the Lucas, Valvoline VR-1 and other off-street oils. Or, you can buy the ZDDP as a liquid additive and add a teaspoon.
I don't know that the ZDDP was the reason Dixie changed their minds, but it seems likely. Professional mowing would be a high-stress application. But if you use this Dixie to do your front yard once a week, then that's an entirely different application and stress level.
Hope this helps.