On the Agristar/Cletodium product label it says that adding ammonium sulfate to the mix is recommended to help in killing grasses more effectively. You may want to consider doing this in the future, this product is very cheap and helps glysophate as well. Just a note, if you add AMS to your mix you need to put the AMS in the water first before adding your other chemicals.
Yes, you use it in addition to crop oil. Water has minerals in it that bind to some of the spray molecules and lock them up. Also, some plants have a similar composition on their leaves that also does the same thing. The AMS binds up these molecules so that the chemical molecules are more available to kill the weeds. AMS is also a sulfur/nitrogen fertilizer, so you are getting your money's worth in fertilizer and making the spray chemicals more effective is a free benefit.
