I like to keep them separate.
Equipment fuel is totally a direct expense. You do not have an equipment "expense" if you are not using it to create "income."
Truck fuel can be more of an indirect expense, especially for owner operators. There are a lot of things you can do with your truck that may not directly create "income," therefore some of your truck expense is the "cost of doing business."
Example: You could spend $400 in fuel to go to a trade show in Orlando, and happen to take your children to Disney while you are there. That $400 expense has nothing to do with how profitable your actual jobs are.