No_Thanku is right,
First, go to oil and max it out all the way to 20, this will also make water at 20 being that 20 is the max
Second, go over to water pressure, go
-1
+1
-1
+1
This will set water back to 20, while there have been 4 changes to it technically, so the oil is now at 16.
Lastly, go to water and from 20, make it go to 13, and this will in turn make oil go to 9, and then the puzzle will be complete