ExxonMobil Univis Series, Hydraulic Fluid

[solocraftvs]

Hello,
I'm curious if anyone out there knows what the base oil is for the Univis J26 (Univis HVI)? They have a 26 and 13 cSt version. Or...more specifically...what are they doing to get such incredibly high viscosity indexes?
Univis Series

 

[supercity]

Mineral oil and VI improver, says the SDS

300+ VI Hydraulic oils are popular for cold condition applications and Aviation.

 

[bruce381]

low vis oil like similar to keosene with a large amunt of VII 10-15%
there are some oils like this in the 500 VI range