for how many years can you store engine oil?

In it's original sealed container should be good for 5-10 years, depending on storage conditions. Once opened I would use it within a year for oil changes, a bit longer for top ups. you could extend that a bit if you are meticulous about resealing the containers.

The oil manufacturers, who are the engineers of the stuff, say it is good up to 5 years unopened.

About 6 months if opened.

I will assume that the 5 year limit has to do with the various additives in it.

Look in your car service book or the stand by mains power plant books and they say that on low usage engines the oil should be changed yearly something to do with absorbing water they say

Oil in a sealed container will last years.

Oil is already hundreds of million of Years old. Its maid from.dinosaurs.remains. and if you know about the dinosaurs, you'll know that they have been dead for hundred of millions of year's.

You can store oil as long as you wish. Its already millions of years old, a couple of more years wont matter.

Forever.

I wouldn't use it if over 500 years old.

If sealed and stored properly, indefinitely.