https://www.machinerylubrication.com/Read/28956/lubricant-viscosity-indexHigh or Low Viscosity Index needed?
If conditions are not constant (variable loads, variable speeds, variable ambient temperatures, etc.), then there is a need for not only the optimum viscosity but also a high viscosity index to stabilize the optimum viscosity. The more variable the conditions, the greater the need for high VI oils.
Also, keep in mind that for a great many machines there has been no past experimental or theoretical effort to identify the optimum viscosity. Viscosity selection is more of a wild guess. This too calls for a high VI lubricant.
A lubricant may merit having a high VI for one or more of the following reasons:
The optimum viscosity is not known
Varying loads and speeds exist
Varying ambient temperatures exist
To boost energy efficiency
To boost oil service life (lower average temperature)
To boost machine service life (fewer repairs and downtime)
Cheaper, lower VI lubricants may make sense if:
Speeds and loads are constant
Temperature is constant (constant ambient temperature or a heat exchanger is in use)
The optimum viscosity at the operating temperature is known and is consistently achieved
For 75w90 oils;
checking mobil 1 gear oil, it actually has a low VI and according to the above is not good for valkyrie final drives which needs a high VI.
Mobil 1 syn gear oil has a VI of 146.
https://www.mobil.com/en/lubricants/for-personal-vehicles/our-products/products/mobil-1-syn-gear-lube-ls-75w-90/checking some others, Valvoline syn has a VI of 150.
https://sharena21.springcm.com/Public/Document/18452/3e467451-fe75-e711-9c10-ac162d889bd3/cf662252-0bbd-e711-9c12-ac162d889bd1Peak Auto syn has VI of 162 and has the lowest cost.
https://peakauto.com/wp-content/uploads/2017/01/PEAK_Gear_Lubricant_Oil_75W90_75W-140_Oil_Spec_Sheet_2016.pdfAmsoil severe gear has VI of 166.
https://amsoilcontent.com/ams/lit/databulletins/g2043.pdfAmsoil long life has VI of 143.
https://amsoilcontent.com/ams/lit/databulletins/g2042.pdfThis is what I've been using for years in my vehicles. I'll be changing over to Peak from now on.
interesting that the article considers a Low VI as cheap oil.
