Depends on how far you want to go;
When i raced a TZ250 i used to mix the correct fuel /oil mixture in a bucket with a whisk to ensure it was all mixed correctly.
I Never left any fuel in the tank after a meeting as it is possible for the oil to seperate and stick to the fuel tank!! Used fresh fuel each meeting.

Probably way over the top but never had any engine reliability problems!
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MC21 300
Aprilia RS250 track bike