Oil is an important part of keeping an motor vehicle running. And, basics of oil have evolved over the years. A key measure is “viscosity” which indicates the thickness of oil at various temperatures. The “thickness” factor refers to how fast the oil will flow at certain temperatures. Viscosity is commonly referred to as “weight”.
For reference, the higher numbers mean thicker/slower flowing oil. In short, a 5W-30 oil flows faster than a 10W-40 oil. Generally, using a faster flowing oil (5W-30) in colder temperatures is desirable because more oil flows to internal engine parts for critical lubrication. This provides faster starting and reduced wear and tear on your engine.
Adding to the complexity of the proper engine oil is the option of synthetic verses regular oils. Synthetic oils adhere to the same industry standards as regular motor oils. The flow ability is the same. Synthetic oils have an advantage better lubrication properties at higher temperatures.
Like motor engine oil, gear oil has rating standards like GL-3, GL-4, and GL-5. The higher the number, the more lubricity the gear oil has, but also the more Extreme Pressure (EP) additives. Viscosity also applies to gear oil.
The function of an EP additive is to prevent adhesive wear and protect the components when the lubricating oil can no longer provide the necessary film thickness due to pressure and temperature factors.
Boron, chlorine, phosphorus, and sulfur are the most common EP additive types. They are activated by reacting with the metal surface when the temperatures are elevated due to the extreme pressure.
The main differences between GL-3, GL-4, and GL-5 are the EP additives. The EP additives handle closer tolerances between gears but the additives are more durable than the gears, causing a slow chipping away at the metals.
Each transmission has a recommended GL rating to be used depending on the type of materials within the transmission. In general, if GL-4 oil is also rated for GL-3, this means it doesn't have a worrying amount of additives. If it's a GL-4 bottle without the GL-3 rating, it should not be use it in a transmission calling for GL-3.
Automatic Transmission Fluid (ATF) is a simpler comparison when viewing ATF-4 and ATF-3. ATF-, developed to replace ATF-3 applications, degrade much slower under heat and pressure factors. A transmission with ATF-3 might experience some shudder, rough shifting, around the 30k-50k mile range. The same transmission with ATF-4 shouldn't experience similar shudder until 80-100k miles, depending on driving conditions.
In general, ATF-4 is a complete replacement for ATF-3. Synthetic motor oils can replace regular motor oils. Gear oils should be considered application specific.
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