Okay, I think I've got it figured out.
You want to create your own global turn script evaluator that will add a certain number of liters of fuel needed for each type of transition between road types (turn). So, the fuel usage on the edges will stay the same as you have them now, but each transition between edges can add some value, based on the way you want to calculate it.
First, when you create your network dataset, make sure you click the button that tells it you want to model global turns.
Next, set up the global turn evaluator, as follows:
- In the network dataset properties window, go to the Attributes tab.
- For your fuel usage attribute, click Evaluators.
- After the evaluators window pops up, go to the Default Values tab.
- In the Type column next to Turn, click to get a drop-down menu where you can select "Script".
- Click the Evaluator Properties button on the right (looks like a finger pointing at a piece of paper). A dialog will appear where you can write a simple Python or VBScript that says how the solvers should calculate the fuel usage for turns in your network. You should be able to use some if statements to give it a different value based on the road types of the fromEdge and toEdge.