Home mechanics have been pouring vegetable oil into engines for years, sometimes in the fuel tank, sometimes straight into the crankcase, and often with a camera rolling. The appeal is obvious: a cheap pantry staple that might double as fuel or lubricant sounds like a shortcut around high pump prices and pricey synthetic oil. The reality, as testing and research now make clear, is far more complicated, and in most cases far more expensive, than a jug of canola from the supermarket.
After sifting through backyard experiments and formal engineering studies, I found a consistent pattern. Vegetable oil can be coaxed into working as fuel in specific diesel setups, and chemically modified versions can become serious biofuels and even specialty lubricants. But when it is poured straight into an engine in place of motor oil, or dumped into an unmodified car as fuel, the results range from underwhelming to catastrophic.
What backyard tests actually show when oil goes in the crankcase
The most eye catching experiments involve people draining their crankcase and refilling it with cooking oil to see how long the engine survives. In one widely shared test, a creator identified as Jan documented how a small engine ran surprisingly smoothly at first with vegetable oil in place of motor oil, the idle sounding normal and the temperature gauge staying calm for a short period. Another video, posted in Feb, shows a similar setup where the host pours vegetable oil into the crankcase and notes that it initially seems like a “pretty decent substitute” as the engine revs without obvious drama.
Those early minutes are deceptive. As Jan kept running the engine, the thicker vegetable oil struggled to circulate through tight passages and around fast moving bearings, a problem that became obvious once the engine was opened and the previously mentioned difference in viscosity showed up as darkened, stressed metal surfaces coated in a gummy film of degraded oil. In the Feb experiment, the host eventually concedes that the oil only behaves acceptably “for a short period of time,” a verdict that lines up with the way these tests tend to end: with accelerated wear, bearing damage, or a seized component once the improvised lubricant overheats and breaks down.
Why vegetable oil fails as a long term lubricant
On paper, vegetable oil looks like it might work in an engine. It is a fatty liquid with some lubricating ability, and in controlled lab conditions certain plant based esters can show promising friction and wear numbers. The problem is that real engines are hot, dirty, and oxygen rich, and that is where the chemistry turns against you. Detailed work on waste vegetable oil based esters notes that, on the other hand, vegetable oils have several disadvantages, including poor oxidative and thermal stability due to unsaturated bonds that readily react at high temperature, which makes them prone to thickening, sludge formation, and varnish when used directly as lubricants.
That instability is not just a theoretical concern. When vegetable oil is heated repeatedly in an engine, it polymerizes and forms sticky deposits that can clog narrow oil galleries, hydraulic lifters, and piston cooling jets. Researchers looking at plant based lubricants have had to modify the base oil, for example through epoxidation of methyl esters from waste vegetable oil by heterogeneous catalysts like WO3/Al2O3, precisely because unmodified oils cannot survive the combination of heat, oxygen, and mechanical shear inside a modern powertrain. In other words, the same chemistry that makes a frying pan go gummy after too many uses without cleaning will do the same thing inside your crankcase, only with far higher stakes.
Fuel versus lubricant: two very different jobs
Part of the confusion comes from the fact that vegetable oil can, in some circumstances, be used as fuel, particularly in diesel engines, which has led some drivers to assume it might also stand in for motor oil. A comprehensive review of the use of vegetable oils as I.C. engine fuels explains that the effect of the fuel on engine performance and emissions in both spark ignition and compression ignition engines depends heavily on viscosity, combustion characteristics, and how the fuel interacts with materials over time. That same body of research stresses that what works in the combustion chamber does not automatically translate to the lubrication system, which has its own temperature profile, flow requirements, and contamination challenges.
Even within the fuel world, vegetable oil behaves very differently from conventional diesel. Technical guidance on alternative fuels points out that vegetable oil is thicker and has a higher viscosity than diesel fuel, which makes it harder for injectors and pumps to atomize and spray the thicker fuel. That is why systems designed for straight vegetable oil typically rely on preheating and specialized hardware to thin the oil before it reaches the injectors. Motor oil, by contrast, is engineered to maintain a very specific viscosity over a wide temperature range, to suspend combustion byproducts, and to protect against corrosion, none of which straight cooking oil is formulated to do.
When vegetable oil actually can power a car
There is one area where vegetable oil has a proven track record: as a fuel for certain diesel engines, especially older designs with mechanical injection. Reference material on vegetable oil fuel notes that vegetable oil can be used as an alternative fuel in diesel engines and in heating oil burners when it is properly heated and, in many cases, when the engine is modified to handle its higher viscosity and different combustion behavior. In these setups, the oil is not lubricating bearings or camshafts, it is being burned in the cylinders, which is a very different role.
Owners of specific models have reported success with this approach. On enthusiast forums, drivers point out that, however the VW, Audi, Skoda 4 cylinder diesel cars with the Bosch VP37 type injector pump are direct injection yet also veg tolerant, provided the system is set up correctly and the engine is not run on cold vegetable oil for more than a few seconds from cold. Consumer testing has described how a Jetta was modified to run on used cooking oil using a conversion kit from Greasecar Vegetable Fuel Systems, which added separate tanks, heaters, and plumbing so the car could start and shut down on regular diesel while using hot vegetable oil only under steady running. These examples show that with the right hardware and operating discipline, vegetable oil can be a workable fuel, but they do not suggest it can replace engine oil.
The biodiesel and city scale waste oil story
Beyond individual tinkerers, entire fleets have experimented with turning used cooking oil into energy, but almost always by transforming it into biodiesel rather than pouring it straight into engines. One city level program highlighted how waste oil from restaurants was collected and processed so that vehicles could run on biodiesel from waste cooking oil, with officials stressing that, normally, to adapt a diesel vehicle you would need to check what types of fuel your engine can take and whether the fuel needs to be converted into biodiesel before it is put into the engine, thereby reducing its viscosity. That conversion step, which chemically alters the oil into methyl esters, is crucial to making it behave more like standard diesel in pumps and injectors.
On the consumer side, some drivers have taken a more DIY route, collecting fryer grease from local restaurants and filtering it for use in personal vehicles. One driver told a local station that, after basic filtering, that is it, but if you are driving a large vehicle like a big Ford pickup, or even a Mercedes diesel or a Volkswag diesel, you still need to install a proper system, often sourced from a site called greasecar.com, to heat and manage the fuel. These stories underline a key point: even when vegetable oil is the energy source, it is almost always processed, heated, and carefully managed, not simply poured into the tank or crankcase as a direct substitute for the fluids the vehicle was designed to use.
What happens when you pour cooking oil where motor oil should go
The most dramatic failures come when people ignore that distinction and treat vegetable oil as if it were interchangeable with engine oil. In one widely viewed clip, a host identified as Jan fills an engine with cooking oil and notes that, at first, the engine is not skipping and nothing feels different whatsoever, at least to the ear. That calm does not last. As the test continues, the oil heats up, thins unpredictably, and begins to oxidize, losing its lubricating film and allowing metal to rub on metal in ways that conventional oil is specifically engineered to prevent.
Technical explainers on what happens when you feed food products to engines make the same point in less theatrical terms. One analysis bluntly states that cooking oil will just burn up in your engine when used as a lubricant, with the key concern being heat tolerance and the lack of additives that motor oil relies on to handle extreme temperatures and contamination. Without detergents, anti wear agents, and dispersants, cooking oil cannot keep soot and microscopic metal particles in suspension, so they settle out as sludge or abrasive paste. Over time, that combination of burnt oil and debris can clog oil control rings, starve bearings, and turn a running engine into a very expensive paperweight.
The science on vegetable oil as fuel, and why “short period” matters
Researchers have spent decades probing whether vegetable oil based fuels can work in internal combustion engines, and their conclusions are nuanced. A technical abstract notes that a number of researchers have investigated the feasibility of vegetable oil based fuels and that most of them have concluded that such fuels can be used safely for a short period of time, especially in compression ignition engines, but that long term use raises concerns about deposits, injector coking, and lubrication oil dilution. That phrase, “safely for a short period of time,” is doing a lot of work, and it reflects the difference between a one off demonstration and a maintenance schedule measured in years.
Detailed chapters on rapeseed oil and other biofuels, including section 4.2 on fuel properties with reference [36], explain that vegetable oils, also known as biofuels, are preferred for use as fuel rather than diesel fuel in some contexts due to their renewable nature and certain combustion characteristics, but they also emphasize the need for transesterification, blending, or engine modifications to manage viscosity and cold flow. Another technical overview of castor and other plant oils notes that it (vegetable oils) has properties similar to diesel in some respects, however vegetable oil cannot be used directly in the engine, but can be converted to biodiesel or used in dual fuel systems to avoid problems like injector as well as gum deposition [4]. The pattern is clear: even in the fuel role, straight vegetable oil is a compromise that engineers work around, not a drop in replacement.
Why your owner’s manual still says “no”
Automakers and service shops are blunt about what should and should not go into an engine. Guidance on oil changes makes it clear that the answer is no when drivers ask if they can substitute fluids that the engine was not designed for, stressing that it (synthetic oil) is not at all recommended in vehicles built for conventional oil without proper specification matching because your vehicle’s engine was not designed to run on the wrong viscosity or additive package, and that ignoring those requirements can leave you holding a very expensive bill. If that is the stance on different types of petroleum based oil that still meet industry standards, it gives a sense of how far outside the design envelope straight cooking oil would be.
Fuel system advice is just as strict. Consumer facing explainers on alternative fuels warn that, due to the oil being so thick and sticky, it will not flow properly through the engine and it will not burn efficiently if you simply pour supermarket vegetable oil into the tank of a modern car, which can cause the engine to burn out or stall. That warning applies even before you get to the question of lubrication, where the tolerances are tighter and the consequences of failure more immediate. In practice, if a manufacturer has not explicitly approved a given fuel or lubricant, using it is a gamble with the most expensive component in the vehicle as the stake.
Where research is heading, and what drivers should actually do
None of this means vegetable oil has no future in engines. On the contrary, ongoing work on plant based esters and additives is trying to harness the renewable appeal of bio based oils while engineering around their weaknesses. Studies on epoxidation and other chemical modifications are aimed at improving oxidative and thermal stability so that bio derived lubricants can survive high temperature operation without forming sludge, and some niche applications already use carefully formulated vegetable based fluids where environmental spill risk is a bigger concern than extreme performance. The key is that these products are engineered from the molecule up, not scooped from a deep fryer.
For everyday drivers, the practical takeaway is straightforward. If you are interested in cutting your carbon footprint or fuel bill, there are legitimate paths, from certified biodiesel blends at the pump to professionally installed straight vegetable oil systems on compatible diesel vehicles, backed by data on engine performance and emissions from sources such as the effect of the fuel on engine performance and emissions in compression ignition engines. What is not on the table, at least if you care about the longevity of your car, is treating the bottle of canola in your kitchen as a substitute for motor oil. The experiments are entertaining, and the science behind them is fascinating, but the results are in: vegetable oil belongs in the pan or, at most, in a carefully engineered fuel system, not in the crankcase of your daily driver.
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