In light of the huge operating speed of the geared tubofan gearbox gearing down the fan speed, what Oil thickness is used for the geared turbofan jet engine gearbox and does it use use regular gearbox oil used in cars and trucks, also in light of the huge difference in operating speeds of these gearboxes?
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 & 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.
I don't think oils for this application are classified by weight. The specification for Mobil Jet Oil II, for example, which is one of the approved lubricants for this application, is quite a bit more complex.