Did some tests the other day on oxidized U.S. R15c, R6c, R30c, and R135.
Common hydrogen peroxide solution that you can buy from any pharmacy for under a buck, applied via Q-tip, and then a rinse in water to remove the peroxide.
I tried a relative wide variety of stamps with varying states of oxidation. If the oxidation has gone too far, the peroxide won't fully bring the stamp back, but on stamps that are "just starting to turn" it has wonderful restorative effects.
Below is a stamp that shows the before and after states. I only applied the peroxide to the bottom half of the stamp.
