Solution : https://service.sap.com/sap/support/notes/620698 (SAP Service marketplace login required)
Key words : 
-> petroleum measurement standards -> quantity conversion interface, oil quantity conversion box valid, oil quantity conversion dialog box, -oil specific gain/loss functions, additional gain/loss line item, gain/loss postings tolerances checks, relevant conversion group -> tolerance levels, industry solution oil& gas, gain/loss posting values, gain/loss tolerancesmight conflict
Related Notes : 
       
| 652834 | Additional Quantity Handling: Manual Changes and Effects | 
| 577190 | QCI: Enhancements with SAP Oil & Gas 4.6C Service Pack 2 | 
| 508765 | Two-Step Plant to Plant Transfers with Gain/Loss |