Op-amp slew rate over frequency
Slew rate is the maximum dv/dt the op amp can produce at its output, and it doesn't depend on frequency. 

This means that at high frequencies, the output will be distorted. The distortion happens when the slew rate is not enough to support the dv/dt required to accurately produce a sinusoidal output. Since the dv/dt of the sine wave depends on both the frequency and the amplitude, you can reduce the distortion by reducing either one.

Edit


  what frequency should I use to measure my slew rate?


As you found, you can measure the slew rate at any frequency that is fast enough to exercise the slew rate limiting behavior. As discussed above, the minimum frequency to see the slew rate also depends on the input amplitude. You could also use a step input rather than a sinusoid to measure the slew rate.