Various available fluids found online that may be appropriate for hydraulics:
 

Pentosin CHF 11S:

• Viscosity (40C): 18.7mm2/s
• Viscosity index: 313
• SDS indicates 68649-11-6 1-decene dimer hydrotreated (also used as a skin product, grinding oil, etc.)
• Recommended on MTBR forums many times

OEM power steering fluid (European):

• Contains highly-refined mineral oils
• Made by Recochem Inc code: 15-851OEM
• SDS has no useful information, other than not hazardous. So maybe does not have major additives?

OEM power steering fluid (Asian):

• code: 15-891OEM, probably the same as above product

Prestone European power steering fluid:

• Stated as alternative to CHF 11s, or CHF202 (which is partial synthetic)
• Contains "seal conditioners and anti-corrosion additives"
• Ingredients: different base oil, and a few additives (zinc and anti-wear)

FEBI hydraulic fluid M6162:

• Viscosity (40C): 16mm2/s
• Seems to be mineral based, not synthetic?

Synthetic DOT3 (eg lucas oil synthetic):

• Different base oil (9004-74-4), could be more hazardous

Advantage of synthetic:

• Wider temperature range
• Maybe more compatible with materials? (seems to not be the case)

Viscosity index: lower = change more with temperature, higher = change less.

Viscosity: higher = thicker. 

Kinematic viscosity: how fast fluid is moving when force applied.

Links:

https://www.pinkbike.com/forum/listcomments/?threadid=140344&pagenum=3

https://forums.mtbr.com/brake-time/mystik-mineral-hydraulic-oil-brakes-1070783.html

https://www.ncbi.nlm.nih.gov/books/NBK304428/ (mineral oils study)