What Oil thickness is used for the geared turbofan jet engine gearbox
The more regular specification for such oils is given for the viscosity in centistokes versus temperature, as an example the Mobil Jet™ Oil 387 lubricant that can be used on Pratt &amp; Whitney PurePower® Geared Turbofan™ PW1900G engines has a viscosity of about 5 cSt at 100°C, and about 25cSt at 40°C

Other parameters are extremely important such as the flash point temperature, the fire point temperature and the autogenous ignition temperature

Also the Total Acid Number is an important parameter for engines oil.

Please refer to the following website for more details about above mentioned oil specifications.

https://www.exxonmobil.com/english-US/Aviation/pds/GLXXMobil-Jet-Oil-387