How many Kms to wear down BHP?

[Ambrose#47]

Hi,

I need to know how many kms I have to do to wear down BHP after an oil change increased my BHP beyond league regulations?
Any ideas?
And also does it decrease in Practice mode?

 

[Ambrose#47]

For anyone who is interested about 200Kms brought it down 7 Bhp.
Just ran it B- Spec in the Clubman Cup championship 3-4 times.