Ruling on disposing of samples after testing them
Question

                My wife works in a petroleum store. They test the samples of petrol by taking an amount of about 1.5 liters of petrol. These quantities are transferred by cars, in tanks. Is it permissible for me to take these samples after they finish testing them? I mean to put such samples into my car’s tank and use them. Note that the samples are not returned after testing.
Praise be to Allah.If the samples are to be 
    returned after examining them, then they belong to them and it is not 
    permissible to anyone to take any of them without their permission, because 
    the Prophet  (peace and blessings of Allaah be upon him) said: “The 
    wealth of a man is not permissible unless he gives it willingly.” Narrated 
    by Ahmad (20172); classed as saheeh by al-Albaani in Saheeh al-Jaami’ 
    (7662). 
But if these samples are 
    not returned to the owners because they no longer want them, rather they may 
    be consulted as to do with them, then if they allow those who take them to 
    dispose of them, it is permissible for the employees to take them and 
    dispose of them. 
It is permissible – even 
    obligatory – for them to take them if they have been told to get rid of 
    them, because Islam forbids wasting wealth and enjoins being grateful for 
    blessings, and wasting them is contrary to these two commands. 
It seems – and Allaah knows 
    best – that the owners of these samples are not asking for them and they 
    regard them as no longer belonging to them; if that is the case then there 
    is nothing wrong with anyone taking them, and it is like the wealth that a 
    person leaves because he no longer wants it, so the one who takes it becomes 
    its owner, as the scholars said. 
See: al-Mughni 
    (5/347). 
And Allaah knows best.