There is no upside for this request worth considering.
This is comparable with a murder taking place in your home and the police wanting to talk to you. In 100% of the cases, particularly if you are innocent, you should hire a lawyer to represent you. You know you didn't kill the victim but the police can find a way to use your actions to make you look guilty.
Giving up a fuel sample is exactly the same thing. You bought fuel on a good faith basis from everybody you purchased from. If there was some contaminant in your fuel system, you will be blamed. If the fuel is not contaminated but their analysis procedure is flawed, you will be blamed. If the dealer has sinister motives, you will be blamed.
You're not going to hire a lawyer but you should have the good sense to say no.