according to Marzocchi, top out is remedied by increasing oil viscosity. Obviously there could be other factors, but by increasing the oil's viscosity, it will keep the piston from moving back upwards so quickly once the coil and air springs reach their maximum compression and start to rebound. Installing a softer coil spring and/or reducing air pressure in the air cartridge will allow more downward travel and won't rebound so harshly. Going with a lower weight oil will actually contribute to top-out, so I wouldn't recommend going that direction with things. You'll know you've gone overboard on the rebound damping when the fork starts to "pack up" (stays down and rebounds very slowly) over multiple smaller impacts.