Drivers Education

Can You Use Engine Oil in Engine Gearbox?

Circumstances can make you ask a typical question like “can you use oil in engine gearbox?” Many have used engine oil in their gearbox, either mistakenly or consciously, when there are no readily available options. But is it OK for your gearbox to contain engine oil?

Somehow, it’s okay to use engine oil in the gearbox for some vehicles, especially motorcycles. Besides, some motorcycles use similar oil for gearbox and engine. Nonetheless, automakers do not recommend using non-transmission oil for gearboxes.

Meanwhile, if you poured motor oil into your transmission by mistake, do not drive the car, except your manufacturer recommends it. But you must find out, though; you may use your vehicle model forums or refer to the owner’s manual.

If circumstances force you to drive with motor oil in the gearbox, consider applying a fluid stabilizer before driving. But if you used full synthetic, you probably should not drive at all until engine oil is flushed.

Is engine oil and gearbox oil the same?

The simple answer is “No”. While engine oil is designed for the car engine, transmission or gearbox oil is designed for your gearbox. Both oils appear to be similar, but they function differently, and manufacturers do not recommend misusing one for the other. Nevertheless, transmission and engine oils contain some similar properties.

One of both oils is designed to be viscous, while the other is supposed to be thinner. The oil with higher viscosity absorbs more heat while retaining its volume, and the other is designed not to thicken in cold temperatures.

Though people use both engine oil and transmission oil interchangeably, it is not recommended if you care about your degrading your vehicle.

Can You Use Oil in Engine Gearbox?

Honestly, you are not supposed to use engine oil in your gearbox; the gearbox requires transmission oil. Besides, most gearboxes need special lubricants to enhance a vehicle transmission system. And before you add engine oil to the engine or transmission oil to the gearbox, it must meet the automaker’s requirement or recommendation.

Gearbox gears often heat up, causing load, wear and tear, and high pressure, especially with the fact that the RPMs hit thousands. Of course, it is the gearbox or transmission oil that preserves the vehicle from physical and thermal damages by reducing friction. Gears also remain clean with proper transmission oil in place.

A typical lubricant contains the chemical and physical properties required for your vehicle. So, it is obvious that by applying a non-recommended oil in the gearbox, you would be messing with the vehicle system.

Your car gears must remain lubricated with splash lubrication, and is recommended to dip one gear in lubricant for optimal performance. As mentioned above, the gear apartment dissipates, resulting in extreme heat, which engine oil is not designed to handle.

Instead of driving with engine oil in transmission, gearbox oil, containing 80-90 SAE (Society of Automotive Engineers) lubricant standard. Of course, this level of lubricant is thicker than typical engine oil.

Moreover, when you use transmission oil thicker than 90, you’d experience motion resistance within the gearbox. Oil with a thinner lubricant content would typically leak from the seals at the ends of the shaft, wetting the clutch plate, which causes friction or decreases transmission efficiency.

What are the exceptions?

Exceptions apply but are never recommended by the manufacturers. So, you may use engine oil in the gearbox, but it depends on certain factors. Typical motor oil contains detergents, which is bad for your gearbox. Motor or engine oil also contains higher anti-oxidants to accommodate higher temperatures, which is less viscous sometimes.

In a Nissan 5-speed trans and Toyota, for instance, 75W-85 are common gearbox oils, which is similar to 5W-20 in engine oils. Transmission oil and motor oil require anti-wear additives or extreme pressure.

Some time ago, however, some gear oils contained sulfur/phosphorous additives to enhance EP protection and had more acid content. Today, most vehicle transmission oils feature similar formulations as motor oils but less slippery and quite higher on EP.

Meanwhile, most motorcycles use one oil for the gearbox and engine since the oil has a clutch pack and compact design. The oil must, however, be rated JASO MA2 to ensure reliable clutch grabbing ability, and it works for automatic transmissions.

Also, a typical manual gearbox Classic Mini uses either engine or gearbox oil, which means you can apply engine oil in the gearbox.

Gearbox and Engine Oil Myth

Don’t be scared if you poured engine oil in your gearbox; let’s have some fact check. Most drivers believe that transmission oil is heavier than motor oil because motor oil is designed between 0w30 to 15W45 while transmission oil is at about 75W85. Well, it is a myth that gearbox oil is thicker because both oils contain similar viscosity.

The major difference between engine oil and gearbox oil is the additives. A typical engine or motor oil contains additives, including dispersants and detergents, to fight by-product chemicals released from the vehicle gas ignition.

Engine oil contains these additives for bearing lubrication with hydrodynamic film and for the oil pump in the internal combustion engine.

Meanwhile, gearbox oil contains high-pressure additives that are not necessary for the engine. The drivetrain comprises several components with ferrous material. So, it requires appropriate lubricant to prevent corrosion and rust, whereas corrosion and rust are not common in a car engine.

What happens if I put engine oil in my transmission?

What happens when you put engine oil in your transmission is that it causes overheating and poor lubrication that may result in inefficient transmission or transmission failure. And it does not matter whether you added the oil mistakenly or not. What’s worse? The damage or degradation might be irreversible even after you flush the transmission.

Typical transmission oil is viscous, and when it enters the oil pump, extreme pressure is required for pumping.

Meanwhile, there is no cylinder bore temperatures rating for transmission oils; it means that they will burn faster, losing lubrication properties quickly. Transmission oil is not also rated for bushes and bearings, which means the bearings are bushes degrade or damage faster. And your engine is not safe.

The best option is to get the right fluid or recommended oil for your car. Depending on your vehicle manufacturer, the recommendation could be 680 VG (viscosity grade).

Final Thoughts

Honestly, it is better to follow your vehicle manufacturer’s recommendation when choosing oil for your gearbox. The circumstance might force you to follow measures such as mixing oils or applying additives to the oil before pouring in the gearbox.

Understand that a typical lubricant or oil has specific chemical and physical properties that improve your vehicle performance. So, when you apply the oil wrongly, it affects the performance of the car, and you might be unable to reverse the action.

Yes, most two-wheelers or motorcycles use motor oil in the gearbox, but a car functions differently and requires the recommended oil for the gearbox to perform optimally.

In terms of detergent properties, heat transfer, and adhesive, motor oil is superior to transmission oil but avoid using similar viscosity. You may apply engine oil with lower 10W30 viscosity, though. But the rule of thumb is to use the manufacturer-recommend transmission oil for transmission.

Latest posts by Bernard Juchli (see all)

Bernard Juchli

Bernard Juchli is an experienced racer, mechanic and team owner who trusts Avon Tyres.Bernard is the lead driver and force behind his Big Dog Garage Race Team. He is the General Manager and Chief Mechanic of Jay Leno’s Garage. Bernard and his crew of seven are responsible for all repairs, restoration and fabrication of Jay’s incredible automobile and motorcycle collection.

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