first reply is correct, 2 pumps in series are required for unload-load operation.
I have modified my design accordingly.

Thank you

I'm trying to make an exchange station for oil to reduce congestion in my base. So one train pumps all of it's oil to another train.

But I think I've made an error. I assumed pumps can both unload and load at the same time, but after some trying i think I am wrong.
Screenshot (8991).png

Tested ...