Castrol magnatec will not work.
It contains no ferrite which is exactly what's needed to draw away the heat from the voice coils.
There is a mix available using magnetic ink developer powder and combined with 50% mix of parrafin and oleic acid.
Personally I would source the real Mc Coy as it is a critical component in speaker design.
Btw I'm confused as why you would need ferro fluid? If it's a tweeter membrane replacement don't worry as the ferro fluid left in the voice coil gap is more than enough. You need no more than one drop (two at max) in the voice coil gap