Why GL5 oils shouldn't be used in a Type 9 Ford gearbox with brass/bronze synchro rings

Using GL-5 gear oil in a Ford Type 9 gearbox with brass or bronze synchro rings can be problematic due to the additives in GL-5 oils.

The primary concern is the presence of extreme-pressure (EP) additives, particularly sulphur and phosphorus compounds, which are added to GL-5 oils to provide better protection for hypoid gears commonly found in rear differentials.

The issue arises because these EP additives can have a corrosive effect on yellow metals like brass and bronze, which are often used in synchro rings.

Over time, the EP additives in GL-5 gear oil can react with and gradually wear away these soft metals, leading to premature wear and damage to the synchro rings.

This can result in poor gear changing performance, increased gearbox noise, and eventually the need to replace the synchro rings.

To prevent this problem, it's recommended to use a GL-4 gear oil in manual transmissions that contain brass or bronze synchro rings.

GL-4 oils have a milder additive package that is less likely to harm yellow metal components. They provide adequate lubrication and protection for manual transmissions without the harmful effects associated with GL-5 oils.

Always refer to your car's gearbox manual and manufacturer recommendations for the correct type of gear oil to use.

As your Ford Type 9 gearbox contains brass or bronze synchro rings, it's best to follow Ford's guidance and use a GL-4-rated gear oil to ensure the longevity and proper functioning of your T9 transmission.

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