My dad had some old Quaker State in an unopened can we found in the back of the garage when they were moving. No telling how old it was. Looked just fine...used it in the lawnmower, but wouldn't put it in my carIIRC some of that stuff was more like 50 years old. There were no signs of age related deterioration.
Ha, running a John Deere from 1990 - still remember buying it; dad replaced his with a self-propelled Honda. Deere smokes a bit, but couldn't complain about it otherwise - 21 years for a piece of yard equipment is fantastic in my book - it usually just got whatever conventional oil was on the shelf in the garage.Been using synthetics in my air cooled back breaking toys as I call them for years. If anything requires synthetics, its an air cooled engine.
They last years longer.