Apply some physics and logic to the equation.
1) whats in your chosen "additive"?
Power steering fluid works well in the respect that some brands have rubber/silicon/whatever in them to preserve auto hoses, but as a bonus aid traction for us guys.
2) Do the "ingredients" (no tasting guys... we're racing not cooking
) "eat" into the tyre to give the grip, or do they layer it?
Layering the tyre with an additive may help a) soften the rubber but also b) not wear the tyre out and the track feeds on the layer if its thick enough?
It could be that golden syrup works well, nice and sticky. Umm... Not able to take it home with you?
Then just leave your car on the track for next week....