Hi Nicolas
The Fleet app has been changed in version 14.
To record refueling, you must use, for example, a service type "Refueling" when creating a new "Service".
Costs are now linked to Services types (that would be like cost types) which can be of the categories "Services" or "Contracts".
You can use "Filters" and "Group By" to create specific views that you need, and save these to favorites to have them at your disposal.
In the new version you can also schedule activities, Log notes, send messages or attach files for each Service or Contract.
The import function is inside the "Favorites Button" and with the right template you can import almost anything in Odoo, the best way is to export what you need, creating a template to be used for the import.
The new version requires some time to adapt to it, but brings advantages.
What are still missing are integrations with other apps like Expenses, Invoicing and Assets for example.
I hope it helps
BR
Ricardo