The aux fans have two speeds, low and high
Low switches on when the A/C pressure gets to a certain limit.
High is on when coolant temp hits 107C
Regardless of the fact that you are using 10w-40, there should still be good pressure at idle.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record