Shimano’s argument is that they know what’s in their fluid, thy have no idea what’s in halfords, citoren, castrol, magura or anyone else’s. There could be additives deliberately designed to swell the seals on old cars slightly and seal up leaks/cracks.
I like my argument from last time:
paid £12 for the big (litre?) bottle 10 years ago and it’s barely 25% used!
That’s a cost of about 25p/year.
Other things costing 25p in biking terms:
25% of an energy gel
Petrol ued in the engine whilst running the portable jetwash.
Spoke nipple
Piece of ginger for baking carrot cake pre ride
A shower at a trail center if you share with 4 other guys (or ladies).
So other stuff might work, but the shimano stuff might work fractionaly better (lower viscocity, gaurenteed not to attack the seals, correct boiling point).
And I don’t buy the “it works in a 3 ton rolls royce” argument, it may well do, but it probably has over a litre of fluid and pads 10mm thick insulating the fluid from the brakes. MTB’s have 1mm thick pads and 20ml of fluid. They also don’t have ABS, or damping unlike either car brakes or suspension.