In the reception area, the same question is often heard from the mechanic: which way to change the oil — through the drain plug or by vacuum extraction through the dipstick tube. The reaction of car owners is often the same. Many are convinced that the only correct option is the classic drain through the plug in the crankcase. It is widely believed that vacuum extraction leaves dirt and metal shavings at the bottom of the engine, which then mixes with the fresh oil.
However, if you carefully consider the design of modern engines and oil pans, it becomes clear that the situation is not so clear-cut.
Why the Drain Plug Isn't Always at the Lowest Point
The idea that the drain plug is located at the lowest point of the crankcase came from the 1990s. Back then, most cars were equipped with stamped steel pans, where the drain hole was indeed located in the deepest part of the design.
Modern engines are designed differently. On many new cars, the pans are made of aluminum or even high-strength plastic. In such designs, the drain plug is often located not at the bottom, but on the side.
In addition, aluminum pans often have a reinforced threaded well inside — a thickening of the metal that protects the thread from damage when the plug is repeatedly unscrewed. This internal protrusion can rise above the bottom of the crankcase by several millimeters.
What Happens During a Traditional Oil Drain
When the plug is unscrewed, the oil begins to flow freely. However, as soon as the liquid level drops to the edge of the internal threaded well, the flow practically stops.
As a result, a small amount of old oil remains at the very bottom of the pan. This is usually about 150–250 milliliters. It is in this part of the liquid that most of the contaminants are contained — soot particles, wear products, and other impurities.
After fresh oil is poured in, the remnants of the old liquid mix with the new composition. As a result, part of the fresh additive package begins to be consumed from the very first kilometers of operation.
How Vacuum Extraction Works
During a vacuum change, the mechanic inserts a special plastic probe into the oil dipstick tube. The end of the probe has a beveled shape so that it does not stick to the surface of the crankcase.
On many modern engines, the dipstick guide tube is designed so that its end is as close as possible to the deepest point of the pan. This allows the device to pump out the oil almost completely.
The vacuum pump creates a vacuum and gradually draws oil out of the crankcase. This allows the liquid to be removed even from hard-to-reach places where a small residue may remain during normal draining.
This method also has additional advantages. For example, there is no need to unscrew the drain plug, which reduces the risk of damaging the thread in the aluminum pan. Restoring a stripped thread can be a complex and costly procedure.
In addition, on many modern engines, the oil filter is located on top under the hood. In this case, a vacuum change allows maintenance to be performed without removing the engine protection and without access to the lower part of the car.
When a Classic Drain Is Still Preferable
Despite the advantages of the vacuum method, there are situations where a traditional drain remains the only possible option.
For example, on some old engines, the dipstick guide tube does not reach the lowest point of the crankcase. In addition, there may be special partitions or anti-surge plates inside the pan that prevent the probe from passing freely.
Such features are found on some old Japanese engines, as well as on classic domestic engines. In such cases, the probe simply rests against the internal elements of the crankcase and cannot reach the bottom of the pan.
As a result, vacuum extraction is not a universal solution for all cars. But for many modern engines, this method has long been considered not a sign of mechanic laziness, but a technologically justified maintenance procedure.