I'm an expert in the field of automotive maintenance, and I'd be happy to provide you with a detailed answer regarding the practice of adding new oil to old oil.

When it comes to motor oil, there are several factors to consider. First and foremost, the type of oil is crucial. There are different grades and weights of oil, and they are not all compatible.
Synthetic oils, conventional oils, and blends of both have different properties and are designed for specific applications. Mixing different types of oil can lead to a reduction in the performance of the oil and can potentially cause damage to your engine.

Viscosity is another key factor. Oil viscosity changes over time due to heat and contamination. As oil ages, it can thicken or become more viscous, which can affect its ability to lubricate engine parts effectively. Adding new oil to old oil can help restore the viscosity to a more optimal level, but this is only beneficial if the new oil has a similar viscosity to the old oil.

The contaminants present in the oil are also an important consideration. Over time, oil can pick up dirt, metal particles, and other contaminants that can cause wear and tear on engine components. While some of these contaminants can be filtered out by the oil filter, others remain in the oil. Adding new oil to old oil can dilute these contaminants, but it's not a substitute for a complete oil change.

The draining process is another aspect to consider. When you change the oil, it's not always possible to remove every last drop of the old oil. Some oil remains in the engine, and when you add new oil, it will mix with the remaining old oil. This is normal and not necessarily a problem, but it does mean that the new oil will be exposed to the contaminants that were in the old oil.

Oil change intervals are specified by the manufacturer for a reason. These intervals are designed to ensure that the oil is changed before it becomes too contaminated or degraded to perform its job effectively. Continuing to change your oil at the specified intervals is crucial for maintaining the health of your engine.

In conclusion, adding new oil to old oil can be acceptable in certain circumstances, but it's not a substitute for a complete oil change. It's important to use the correct type and viscosity of oil for your vehicle, and to follow the manufacturer's recommended oil change intervals. While some mixing of old and new oil is inevitable, it's best to minimize this by performing a thorough oil change, which includes replacing the oil filter as well.

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As long as you are using the same weight oil, it will mix up and you'd never know it. Continue to change your oil at the specified interval and you should be golden. Usually when changing the oil you won't be able to fully drain the old oil anyways and the old and new oils will mix, like it or not.Feb 11, 2016read more >>