Is There a Problem with Adding Engine Oil to Hydraulic Oil?
4 Answers
Adding engine oil to hydraulic oil is problematic and will render the hydraulic oil unusable. Consequences of adding engine oil to hydraulic oil: Mixing a small amount of engine oil into hydraulic oil will alter the viscosity and viscosity-temperature properties of the hydraulic oil, affecting its performance. Quality requirements for hydraulic oil: Suitable viscosity and good viscosity-temperature properties to ensure accurate and responsive power transmission under varying working temperatures, while also ensuring proper lubrication of hydraulic components; excellent rust prevention and oxidation stability to resist degradation under high temperature and pressure conditions, ensuring a long service life; good anti-foaming properties to allow foam generated under continuous mechanical agitation to dissipate quickly, ensuring stable power transmission and preventing accelerated oxidation of the hydraulic oil.
Once when I was repairing my car, I accidentally mixed a bit of engine oil into the hydraulic system. As a result, the steering suddenly became extremely heavy, nearly causing an accident. Engine oil and hydraulic oil are two completely different things. Engine oil is mainly used to lubricate engine components, while hydraulic oil is specifically designed to transmit pressure, such as in power steering systems. If you mix them, their chemical compositions can react, making the hydraulic oil thinner or causing it to foam. This drastically reduces lubrication effectiveness and may also corrode seals, leading to oil leaks or stuck valves. Not only does this damage the equipment, but it can also cause sudden failures while driving, like loss of steering control. If this happens, stop driving immediately and seek help from a repair shop to flush the system. Otherwise, the consequences could be more severe, such as complete failure of the hydraulic pump. When refilling fluids, it's crucial to pay attention to the labels on the bottles and not take shortcuts by mixing them carelessly.
Engine oil and hydraulic oil must never be mixed, as they serve completely different purposes in a vehicle. Engine oil is designed to lubricate the internal components of the engine and must maintain its viscosity under high temperatures. Hydraulic oil, on the other hand, is used to transmit pressure in systems like steering or braking and needs to stably withstand high pressure. If engine oil is added, its additives can conflict with the formulation of the hydraulic oil, reducing lubrication and anti-foaming properties. The result is unstable oil pressure, increased component friction, and potential clogging of small hydraulic lines, which can damage the oil pump or seals. Over time, this can drastically reduce system efficiency and may even require major repairs or part replacements. A friend of mine who works in a workshop has encountered this issue—after mixing, the equipment exhibited abnormal heat and increased noise. The solution is to use a professional cleaning agent to thoroughly flush the system and replace it with fresh hydraulic oil.
From a repair perspective, mixing engine oil with hydraulic oil can cause maintenance costs to skyrocket. In hydraulic systems like power steering, once contaminated with engine oil, oil passages may clog or leak, requiring disassembly for pipeline cleaning, and sometimes replacement of filters or sealing gaskets. A major overhaul costing hundreds to thousands of yuan is not uncommon. Worse yet, if not detected promptly, continued use can accelerate hydraulic pump wear, potentially doubling the repair bill. I've handled similar cases before—a client delayed repairs for days to save money, only to end up with a paralyzed system and unnecessary extra expenses. Stopping the vehicle immediately and having a service station test the oil sample with equipment to diagnose the issue can save significant costs.
