Is it permissible to take surplus fuel samples that are not returned to their original place after being examined by my wife, who works in a petroleum products warehouse, for my personal use in my car?

If the samples are returned to their owners, it is not permissible to take anything from them without their permission. However, if the samples are not returned to their owners by their own desire, then one must refer back to them. If they permit the disposal of the samples, then it is permissible for the employees to take them and dispose of them. It is permissible for them to take them, rather it is obligatory if they are commanded to destroy them, in order to prevent the waste of money and as an act of gratitude for the bounty. What appears to be the case is that the owners of the samples often do not claim them and consider them to have left their possession. In this case, there is no blame upon whoever takes something from them, and they are considered like abandoned property that is owned by whoever takes it.