Sorry, but the whole "MUST USE SHIMANO FLUID!!!" thing is quite frankly bollocks.
Here is why: Hydraulic seals are generally made from PTFE, Nitrile or Viton. They are all fine Mineral based Hydraulic Oils.
I have you Halfords LHM Plus (used on Citreon hydralics for example) in 4 sets of Shimano brakes for Ooooo a decade with no issues.
It's slightly more viscous than the Shimano stuff so bleeding is a bit more tricky.
I wonder if Shimano's statement has anything to do with the HUGE amount they mark up their mineral oil. It certainly reeks of "don't sue me or ask for warranty" if you use "their" fluid.
I would be very interested in what Shimano make their seals out of if they can be affected by using a different brand of mineral oil.
Nitrile gets killed very quickly by DOT. My guess they are Nitrile, hence the only use our fluid to stop idiots using DOT.