Can You Add Engine Oil Directly to a Hot Engine?

[Jameson]

You can add engine oil directly to a hot engine. However, it's important to note that if the oil has been sitting for a long time without the engine being started, replacing the oil before starting can be harmful because the oil film on the cylinder walls hasn't formed yet, which isn't good for reducing wear. Here is more information about engine oil: 1. Engine oil refers to the lubricant specifically used in internal combustion engines. Besides lubrication, it also has functions like cleaning, rust prevention, and cooling. The base oil in today's engine oils is composed of hydrocarbons, poly-alpha-olefins (PAO), and poly-internal-olefins (PIO), all of which are organic compounds consisting solely of carbon and hydrogen. Some premium engine oils may contain up to 20% esters. 2. Engine oil consists of base oil and additives. The base oil is the main component of the lubricant, determining its basic properties, while additives can compensate for and improve the shortcomings of the base oil, giving it new properties. Additives are an important part of engine oil. 3. Almost every car manufacturer recommends the most suitable oil viscosity in the owner's manual. Porsche is the most demanding car manufacturer when it comes to engine oil because it uses air-cooled engines that rely solely on oil for cooling. In its internal technical bulletins, Porsche provides conclusions on every oil submitted for testing (as requested by oil manufacturers). However, the most basic requirement is that owners choose the oil based on the ambient temperature of their usage environment. Taking Shanxi as an example, where the temperature rarely drops below -25°C or exceeds 40°C, 5W-40 or 10W-40 oils are generally suitable within this range. The above are essential facts you need to know when selecting engine oil: first, decide the oil viscosity based on the ambient temperature of your driving environment, and then consider whether to use mineral or synthetic oil based on your budget.

[DelEverly]

I've noticed many friends have the habit of directly opening the oil cap during warm-up, which is actually quite dangerous. After a long-distance drive, the engine temperature can reach 70-80°C. Opening the cap not only exposes you to a wave of heat but also reveals the churning oil inside. Last time at the auto repair shop, I saw a young guy get scalded on the wrist by splashing oil. I recommend waiting 20 minutes after parking and turning off the engine for the oil to cool down before refilling. If you're in a hurry, here's a trick: check the dipstick right after parking at night—the oil level is most accurate when the engine is cold. Top it up if it's low, and the next morning before heading out, start the car to see if any warning lights come on. Adding oil recklessly during prolonged warm-up can also cause plastic components to deform.

[DiMia]

As someone who maintains my car year-round, I recommend avoiding adding oil directly to a hot engine. When the engine is scorching, the oil's fluidity is poor, resembling thick honey, making it easy to overfill during top-ups. I've seen many car owners experience blue smoke after overfilling, as excess oil can enter the combustion chamber and damage the catalytic converter. It's best to check the dipstick when the engine is cold—park on level ground and wait half an hour after turning off the engine before proceeding. Adding oil should be like seasoning soup: add small amounts at a time, such as pouring in 200ml, then start the engine and let it run for two minutes to allow the new and old oil to mix evenly before rechecking. Using a graduated oil container can prevent guesswork errors.

[Frances]

Last week, my neighbor just made a fool of herself by driving her new car to the 4S dealership. She noticed the oil warning light after a midday highway run and immediately unscrewed the oil cap after parking, only for the pressure difference to spray oil all over the engine bay. The oil system is pressurized when the engine is hot, similar to a pressure cooker. Now, I teach my family a three-step method: turn off the engine and wait at least 15 minutes → pull out the dipstick, wipe it clean, and reinsert it → check the level and top up if needed. Special attention is required for some German cars, which need to wait until the onboard computer indicates the temperature has dropped below 50°C. In an absolute emergency, at least use thick gloves to protect the back of your hand and stand to the side, avoiding the direction of the filler port.