Making automotive gear oil

Challenge: Formulators creating axle and transmission fluids for heavy-duty vehicles must achieve a delicate balance: reduce emissions by improving fuel economy through lower viscosity while maintaining – and even improving – fluid durability and wear protection. Oil drain intervals have evolved from 100K KM to the 300K-500K KM range and up to 750K KM, with targets now set at 1M KM. Achieving these goals is especially difficult for heavy-duty vehicles, which must cope with higher shear, loads and pressure, which can lead to higher wear rate compared to passenger vehicles. 

How we can help: By using high-performance synthetic base stocks, innovative lubricant formulators can meet the demand for lower viscosity – currently 15-25 cSt kinematic viscosity (KV) @ 100°C but trending to 10 cSt and even lower – to boost energy efficiency, without sacrificing performance and durability. Through high viscosity index (VI), our advanced synthetic base stocks can help enhance:

• Energy efficiency for improved fuel economy
• Low-temperature performance
• Shear stability for long drain intervals
• Oxidative stability even at high temperatures
• Wear protection for long gear life 

Lower coefficient of friction can help improve energy efficiency. In FZG-FVA 345 efficiency tests, low-viscosity SpectraSyn™ 4 conventional polyalphaolefin (cPAO) demonstrated lower coefficient of friction compared to Group II/II+ and Group III mineral base stocks. All three base stocks were blended roughly the same at KV 100°C.

In mini-traction machine (MTM) testing measuring traction coefficient, a synthetic blend of SpectraSyn 4 cPAO and SpectraSyn Elite 150 mPAO demonstrated improved performance compared to blends using Group II/II+ and Group III. Better traction coefficient can help improve energy efficiency. The Group II (EHC 50), SpectraSyn™ 4 PAO and SpectraSyn Elite™ 150 blend produced similar traction coefficient as the Group III base oil blend to meet different lubrication formulations and requirements. (All blends have similar KV 100°C viscosity, Load 75N, speed 2 m/s, SRR 0-70%.)

This chart shows the basic properties of three SAE 75W-90 gear oils – one made using SpectraSyn Elite™ 150, one using SpectraSyn Elite™ 65 and one using SpectraSyn™ 100. All the other components in the blends are the same. The Brookfield viscosity numbers at -40° C (ASTM D2893 test) show outstanding low-temperature performance capabilities.

Shear stability is taking on greater importance because of increased use of wide-span multigrade lubricants to improve fuel economy and service life. Because automotive gear lubricants typically contain viscosity modifiers that can shear in service, reducing viscosity and potentially leading to increased wear, stay-in-grade performance is part of the SAE J306 standard. The three fully synthetic formulations in this chart do not contain polymers, but the high molecular weight PAO can suffer from shear. We can see that all three blends are very shear stable, remaining within the SAE 75W-90 spec even at extended test hours. These results demonstrate long lubricant life.

Extending drain intervals is increasingly important for commercial vehicles even as heavy-duty applications operate at higher temperatures. Base stocks that can provide improved oxidative stability help finished lubricants last longer and perform better in extreme conditions. In ExxonMobil tests, formulated oils using both SpectraSyn Elite™ mPAO and SpectraSyn™ PAO showed good performance. SpectraSyn Elite™ 65 and 150 provided superior performance.