regarding fuel additives, in all circuit racing series here straight pump or 'circuit' fuel is the only stuff allowed in the tank, (98-100 RON unleaded) and random samples are tested with a hand held tester, and if anything looks arwy, the sample goes off to the lab.
In F1, I believe all fuels supplied must conform to the FIA spec, and are chemically 'fingerprinted' at the start of the season. Any batch used from then on must conform or is deemed illegal.
As to adding anything to the oil, why, when as Molakule mentioned, often special 'brews' are made up by the oil companies for them. The Shell blokes used to do that here, and I believe the Mobil sponsored teams are at the least using M1R, which isn't available over the counter here at all.
Anyway, they are all too savvy to just dump something into the oil without fully dyno testing first, and mostly they use analysis to check on what is going on anyway.
There used to be a great story going around regarding contingency money, STP and Nascar. Anyone that ran a sticker had to use the product and they did, on their throttle linkages, shifter linkages, etc. I was told not a drop ever made it into a sump