Many mower manufacturers will allow the use of 20W-50 motor oil in their transmissions. While motor oil will work in many hydrostatic transmissions, there is a better option.
There’s no question that motor oil will capably work in mower hydrostatic transmissions. The mower will steer and it will be propelled forward and backward. But there are vital differences in the way that motor oil is formulated compared to dedicated hydrostatic transmission fluid. These differences can play a role in long term performance and component life.
The main factor to consider is that motor oil is designed for a completely different operating environment than hydrostatic transmission oil. Engine oils are formulated to deal with combustion and combustion by-products. There are very specific detergents and additives in motor oil that won’t affect performance or protection over the short term. But over time, these motor oil ingredients will limit the oil change interval and this can have an effect on long term component life.
The detergent ingredients in motor oil hold combustion by-products in suspension and prevent deposit formation. These ingredients are vital in an internal combustion engine. But not only are these ingredients not needed in a hydrostatic mower transmission, they can degrade and shear down over time. These degraded detergents can contribute to oil foaming, which can lead to compromised wear control.
Another advantage that a dedicated hydrostatic transmission oil has over motor oil is the ability to fight oxidation and corrosion. The hallmark of a good hydraulic fluid is oxidative stability and strong anti-corrosion chemistry. These attributes lead to longer service life and improved protection during storage.
Hydrostatic transmissions also have gears. The meshing of gears can shear down and thin out engines oils. Hydrostatic transmission oil is designed to withstand gear lubrication to a much greater extent than motor oil. This means that dedicated transmission oil has a superior ability to maintain its original viscosity and provide more reliable wear protection over the long term.
There’s no question that motor oil will work in mower hydrostatic transmissions that allow its use. But a dedicated hydrostatic transmission oil contains specific anti-foaming, anti-rust, anti-oxidation and anti-wear ingredients. This chemistry will allow for much longer service life, consistent performance and maximized transmission life. We recommend the new AMSOIL 20W-50 Zero-Turn Synthetic Hydrostatic Transmission Fluid.