can you put vegetable oil in a car engine

[SanDaniel]

No, you should never put vegetable oil in a car engine. It is not a substitute for motor oil and will cause severe, costly damage. While chemically both are "oils," they are engineered for entirely different purposes. Motor oil is designed to lubricate internal combustion engines, which operate under extreme heat, pressure, and contamination from fuel and soot. Vegetable oil breaks down under these conditions, leading to catastrophic engine failure.

The core issue is viscosity, or how the oil flows. Modern engines have precise viscosity requirements (e.g., 5W-30). Vegetable oil thickens significantly when cold, preventing the engine from starting or causing oil starvation at startup. When hot, it breaks down and becomes too thin, losing its ability to form a protective lubricating film on critical components like piston rings and crankshaft bearings. This results in viscosity breakdown and increased engine wear.

Furthermore, vegetable oil oxidizes and polymerizes under high heat, essentially turning into a sticky, solid varnish or sludge. This sludge clogs oil passages, blocks the oil pickup screen, and renders the oil filter useless. The engine will be starved of lubrication, leading to overheating and seized components.

Characteristic	Conventional Motor Oil (5W-30)	Vegetable Oil (e.g., Canola)	Consequence of Using Vegetable Oil
Viscosity Index	High (~160-180)	Low (~200 but poor stability)	Poor flow at cold start, breakdown at high temp
Thermal Stability	Excellent (withstands > 200°C)	Poor (breaks down ~150-175°C)	Forms sludge and varnish deposits
Lubricity (Film Strength)	Engineered for metal-on-metal contact	Low for engine applications	Rapid wear of bearings, camshafts, pistons
Oxidation Resistance	High (with antioxidant additives)	Very Low	Rapid oil degradation and acid formation
Pour Point	Very Low (-40°C to -30°C)	Relatively High (~-15°C to -10°C)	Engine may not crank or start in cool weather

If you accidentally add a small amount of vegetable oil to a full sump of motor oil, the risk is lower but still present. You should drain and refill with the correct oil immediately. However, running an engine on pure or mostly vegetable oil will almost certainly destroy it. For a safe alternative to conventional oil, consider certified synthetic blends or full synthetic motor oils, which offer superior performance and protection.

[McAdaline]

Don't even think about it. It's a one-way ticket to a dead engine. I learned this the hard way years ago with a lawnmower—it seized up solid after one summer using cheap vegetable oil. Car engines are a thousand times more complex and expensive. The vegetable oil just gums everything up inside. It’s like trying to use syrup instead of water in your radiator. Stick with the oil that’s specifically made for your car. It’s not worth the risk.

[MackenzieLynn]

From a chemical perspective, vegetable oil and petroleum-based motor oil are fundamentally different. Vegetable oils are triglycerides, which are large, complex molecules. Under the high shear forces and temperatures inside an engine, these molecules break down and polymerize. This polymerization reaction creates long-chain molecules that form a thick, insoluble sludge. This sludge blocks critical oil galleries, leading to oil starvation and rapid engine failure due to a lack of lubrication. The oil simply cannot perform its primary job.

[DeEliza]

I get the curiosity, especially with biofuel talk. But there's a huge difference. Biodiesel is a chemically processed fuel made from vegetable oil, designed to run in a diesel engine's combustion chamber. What you're asking is about using it as a lubricant in the crankcase, which is a totally different system. Even in older diesel engines that can run on straight vegetable oil as a fuel, they still require standard motor oil in the crankcase for lubrication. Putting vegetable oil in your oil fill cap is a guaranteed way to turn your engine into a very expensive paperweight.

[VonMicah]

It seems like a thrifty idea, but it's a perfect example of being penny-wise and pound-foolish. A quart of the right motor oil costs around $5 to $10. A replacement engine can cost $4,000 to $8,000 or more. The vegetable oil will quickly break down, leading to sludge that clogs the engine. You'll face a massive repair bill that dwarfs any savings on oil. The only thing you'll be saving is a trip to the auto parts store, followed by a much longer trip to the mechanic's shop. Always use the motor oil grade recommended in your owner's manual.