Hi Ironbark
I have always wondered why the magic 5 year mark - you are probably right - the oil companies just test and ensure 5 years, because by that point its a) better sense to tell the consumer the oil you bought is useless and b) given retailers long enough to clear stocks. By 5 years the oil company will have come out with a new blend and the old one will be off shelves.
I am using oil dated 2012 in briggs at the moment, and have a lot of oil back to 2005ish (most 2015 onwards) that I wont hesitate to use. I am using 2018 oil in my car at the moment
