Is it all right to mix different brands of engine oil, as long as the viscosities are identical? I was told to never do this. Why?

Mixing different types or brands of oil will not benefit your car engine in any way. It is best to stick to the oil recommended in the owner's manual.

Although the viscosity grade may be the same, engine oils from different companies have different formulations.

Besides reducing friction, oils contain additives which help to transfer heat, wash off contaminants, prevent corrosion and withstand extreme pressures without breaking down.

Mixing synthetic and non-synthetic oils could cause some serious problems, including producing undesirable chemicals.

If you need to top up engine oil in an emergency, make sure you do not mix synthetic and non-synthetic oils.

Most cars now run on synthetic oils. So, if the engine is running low, you can just top up with any synthetic lubricant, even if it has a different viscosity.

But it would be advisable to thereafter drain and replenish with fresh oil as soon as you are able to.