if you mean the stuff you put in the tank then forget it.
after 50k miles, the injectors will have a poor flow pattern and should be removed and run through a proper ultrasonic injector cleaner - this will restore the flow pattern and increase max fuel flow, but tends to make the injectors noisy - that was my experience. the car did run a lot smoother though.
although if you run on modern fuel such as optimax, there is a chance they wont need doing - my last car (volvo t5)had done 55k on cheap fuel before i got it.