As the fuel on both the amount to use in a race as well as the substance is limited. Samples of the fuel are taken and examined if they have been altered after the race. It's not in any case been done on the oil.
There is a gap for teams to investigate this area as additives can have a positive influence on the power.
3 to 5 litre of oil is burned during a race so if they can find additives which will not harm the car but will have some catalytic reaction on the fuel there is your solution.
If you then release these additives to the oil from the sump in the part of the race were you need extra power you can have even more control.
Think about additives you would want to add to the fuel but are not allowed to but through the oil could be inserted into the engine.
There is no sample of oil taken from the car from the organisation other then the oil engineers from the team itself. Exxon mobil, shell, etc.
In gas engine technology (my background) additives in specific oil types and or specific engines are developed to increase efficiency but could just as well be engineered to improve power output. The possibility of this theory was confirmed to me by both oil technicians in gas engine oil suppliers as F1 team oil engineers on visits to a view F1 teams around races over the years. I got these "where did he get this" kind of looks bringing this up with all of them.
The devel is in the detail so my guess will be that besides the engine, areodynamics and all the obvious engineering items having made F1, the future will also be influenced by the use of chemists and catalyst specialists.