For modified motors (blue can), not sure about the stock motors (red can). If you look at the box or manual that came with your motor you will notice that the motor with lower turns uses sintered rotor while the motor with more turns uses bonded rotor.

I have a modified LRP 13.5T BL motor that comes with bonded rotor. I bought it brand new factory sealed combo with LRP Sphere TC. The rotor rotates smoothly, but when I upgraded to larger diameter sintered rotor the motor turns like some restrictions when you rotate it. But now my motor runs cooler than before and I can use bigger pinions/ smaller spur gears on my car.