Does mixing engine oil occasionally have any impact?

[DeIan]

Mixing engine oil occasionally does have an impact. Engine oils should not be mixed. The methods to determine when engine oil needs to be changed and the precautions for use are as follows: Methods to determine when engine oil needs to be changed: The simplest way to check the shelf life of engine oil is to observe whether the viscosity of the oil has changed, determine if the smell of the oil has faded, and observe whether the color of the oil has changed. When observing changes in oil color, pay attention to changes in the additives within the oil. Additionally, filter paper or electronic instruments can be used to check whether the shelf life of the engine oil has expired. Precautions for using engine oil: The selection of engine oil should be based on the requirements of the engine. Choose an oil that matches the engine. Oils of different brands should not be mixed. Multi-grade oils should be selected as they offer advantages such as savings, long life, and high efficiency, providing better protection for the engine. The oil level should be slightly below the mark on the dipstick and should not be overfilled.

[LaCayden]

With over a decade of car repair experience, I've seen many similar cases. Occasionally mixing engine oils once isn't a big issue, but it depends on the mixing ratio and vehicle type. Last time, my friend's SUV ran 500 kilometers after maintenance and temporarily added half a liter of a different brand's 5W-30 oil as an emergency measure. Later when I checked his dipstick, I found the oil film color was uniform and the engine noise showed no abnormalities. The key is that the base oil types must be compatible—for example, mixing full synthetic with semi-synthetic is safer than mixing mineral oil with full synthetic. However, after mixing oils, it's best to shorten the next maintenance interval and check for sludge at 3,000 kilometers. Of course, if you notice symptoms like sluggish acceleration or a sudden spike in fuel consumption after mixing, don't hesitate—replace both the oil and oil filter immediately.

[McJackson]

Let me share some practical insights from a seasoned mechanical design veteran. When two types of engine oil have different antioxidant content, flocculent substances may form. I've personally tested mixing 10% of different brands and running 300 km on highways – light brown suspended particles appeared in the oil pan. After mixing oils, closely monitor dashboard warning lights during the first week, especially the oil pressure reading. For temporary mixing, don't exceed 20% of total oil volume, and prioritize products with identical viscosity grades. Last time on the Qinghai-Tibet route, I assisted an SUV that had mixed oils – the drained oil contained 30% more metal particles than usual, so replacing the oil filter afterwards is mandatory. After driving 5,000 km safely, it's best to conduct an oil sample test.

[StPiper]

Those with a chemical engineering background pay more attention to molecular structure compatibility. When mineral oil's paraffins encounter synthetic oil's polyalphaolefins, polarity differences can cause localized variations in oil film thickness. Once in the lab, transmission electron microscopy observed 0.2-micron colloidal particle clusters forming in the crankcase after 10 hours of mixed oil usage. While not a major issue, it's recommended to avoid high-RPM driving after mixing oils. Modern vehicles' variable valve timing systems have high demands on oil cleanliness, so it's best to add half a bottle of oil detergent after mixing. I tested three vehicles with mixed oils, and cold-start wear values all increased by about 15%, but the impact remained minimal within 5,000 kilometers of driving.