Flushing it out won't hurt a thing, fill it till it runs out the fill hole and with the rear tires off the ground, slowly turn them to rotate the gear in the transmission. A helper on the clutch pedal will be needed. Drain and do again till you feel it's clean enough. Use diesel fuel, mineral spirits, lacquer thinner, or even gasoline, just some form of solvent that will dry on its own over time. I would stay away from Purple Power or the other water based cleaners due to the acids they contain. You don't want to hurt the bearings.
