Good day. Thank you for your question.
FMB140 has an integrated CAN data processor, thus it has 2 modes (ALLCAN300 & LVCAN200). Therefore, you may check under the following links, based on which mode you have purchased (you can check with your Sales Manager/Distributor), and find out whether your vehicle is in the supported list and are the fuel level data readable.
For ALLCAN300 mode - https://wiki.teltonika-gps.com/view/ALL-CAN300#Supported_vehicles_list
For LVCAN200 mode - https://wiki.teltonika-gps.com/view/LV-CAN200#Supported_vehicles_list
If your vehicle is not on the list, then you may perform the autoscanning procedure (please refer here for the procedure - https://wiki.teltonika-gps.com/view/FMB140_autoscan_feature_instructions)
Regarding fuel theft, you can use the server end to determine when the fuel level is dropping significantly and have the server alert on this provided the fuel level data is read by the device, alternatively, you may also use a fuel level sensor that supports BLE or analog input to connect to the tracking device.
Please try this out and let me know if it helps.