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  1. Does oil get old like gas? I have oil 10 years old, is it still good?

    2. Do all brands of conventional oil mix? Or do some have a different base?

    3. Is synthetic oil good for car seals, I understand that seal are made to stay good when in conventional oil, will synthetic oil cause seals to loose their quality?
  1. No oil does not “age” in the container, but 10 year old oil is really only appropriate for vehicles made 8-10 years ago, or more.

Newer cars require oil that meets specification “SM”, and using your old oil–which is likely to be specification “SJ”–is not a good idea if your car/truck/streetsweeper/tractor/airplane is fairly new. Unfortunately you left out very vital information regarding the type and age of your vehicle.

  1. All conventional oils are compatible. That idea of different brands of oil not being compatible with each other went out the window sometime in the late '50s, IIRC.

  2. Some early synthetic motor oils were rumored to result in leaks in seals and gaskets. That is another idea whose time has come and gone. Synthetic oil has certain advantages, and one big disadvantage–namely its price. Most cars do not need, and will derive little benefit from synthetic oil. The only situations where you really need to use synthetic oil are if the vehicle’s manufacturer specifies it, or if you live in an area where winter temperatures typically go below 10 degrees Fahrenheit.

You really need to read the Owner’s Manual that came with your mystery vehicle in order to determine the type of oil that is required. If your old oil matches the type and specification of your old oil precisely, you can use that old oil.

[b] another idea whose time has come and gone.[/b] 

 Yea.  There is a lot of advice handed down from grandpa that has been made obsolete many years ago.

I would TREASURE 10 year old motor oil…It still has effective extreme pressure additives that protect steel against steel friction points. (zinc). Todays motor oil is more concerned about protecting the catalytic converter… Yes, any SAE rated motor oil can be safely mixed with any other SAE rated oil, including synthetics…