I test every ferrit I can get.
Some ferrits lookslike the same, size and collor, but they are all differend.
It's allmost impossible to say *you need this ferrit*.
I can only say, go to the store and ask high frequenty ferrits and don't use ferrits within powerleads.
The ferrit must handle at last 1 kHz for the ocsilator if you whant 30.000 rpm.