May be a silly question but how do you go changing from pump fuel to race fuel? do you empty the tank or just wait till it's pretty low then put the race fuel in & hope for the best, also do they mix well?

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.