Is it harmful to mix auto oil weights such as 10-30 with 10-40?

It is generally not recommended to mix different weights of motor oil, such as 10-30 and 10-40. While it is unlikely to cause immediate damage to your engine, it can affect the overall performance and longevity of the engine in the long run. Here's why:

1. Viscosity Differences:

Different oil weights have different viscosities, which refer to the thickness or flow resistance of the oil. Mixing oils with different viscosities can alter the intended properties of the oil and affect its ability to provide proper lubrication to engine components.

2. Additive Packages:

Motor oils contain various additives that serve specific purposes, such as detergents, dispersants, anti-wear agents, and more. These additives are carefully formulated for each specific oil weight and mixing different oils can alter the effectiveness of the additive packages.

3. Performance Compromises:

Mixing oil weights can result in compromised performance compared to using a single recommended weight. It may affect factors such as oil pressure, oil flow, and overall engine efficiency.

4. Engine Wear and Damage:

In extreme cases, mixing oils with different weights can lead to increased engine wear and potential damage to engine components over time.

It is always advisable to use the oil weight recommended by the vehicle manufacturer for optimal engine performance and longevity. Mixing different oil weights should only be done in emergency situations when there is no other option available and you should change the oil as soon as possible. Refer to your vehicle's owner's manual for specific oil recommendations and consult with a qualified mechanic if you have any concerns or questions.