The Opel doubles the resources of the sector its deployed in. so a normal +5ammo +3fuell will turn into +10 +6. If you place it at a fuel point +7, it will be +14 etc
I do not think this still applies. In the patch notes of 30 July 2013, it states:
Opel Blitz Cargo Truck
Previously, the cargo truck was multiplying a sectors value by a factor of 2. This resulted in extreme edge cases where fuel and munition incomes were higher than expected. To resolve this, supply trucks will only add a set amount of resources to sectors.
• A cargo truck setup within a sector now adds 3 fuel and 5 munitions per minute, rather than multiplying the sectors value by 2
• Cargo truck cost from 200 to 300 manpower
The Opel was then tampered with again on 27 June 2014:
Opel Blitz Truck
Cost reduced from 300 to 200
Resource bonuses are now exclusive to the player