Piston slap is when the rings 'slap' or rattle while the motor is running, causing a ticking or click. It is not harmful to the motor, but can be a little noisy.
By top grade I mean the best quality racing 2-stroke you can find, which will be a synthetic, such as Motul, Bardahl and Bel-Ray. Go to a local motorcycle shop and they'll have it. Really good oil will not only reduce piston slap, reduce smoke, but create less ring wear, with better lubrication and performance.