Re: Dipping a Fuel Tank
I suppose if the officer has some good cause to believe you are burning the fuel in a manner you are not entitled to, he can make the search.
The best way to find the answer might be to consult a local attorney who has knowledge of this area of law ... whatever it might be.
Other than that, you might consider refusing to consent to the search and then making a motion to suppress if you are charged with an offense later in court.
A Nor Cal Police Sergeant
"Make mine a double mocha ...
... and a croissant!"
Walk humbly with your God
-- Courageous, by Casting Crowns