I have a 5 litre bottle of oil out back that I was going to top my car up with when I realised it's been out on the sun for about a month, the bottle is opaque and does not let sunlight through (apart from a slit in the front to show the remaining oil level) I’m just curious to know if the sun damages engine oil in any way.
Engine oil is normally kept inside a light-proof steel box (your engine), so you might not find much information on how it's affected by sunlight. However, other industries use lubricating oil that can be exposed to the sun, and I found this report:
UV light is the enemy, and it can affect oil:
On a molecular level, the long chain lubricant hydrocarbon reacts with oxygen in the presence of ultraviolet (UV) radiation from the sun.
This image shows some oil before and after being exposed to the sun for 5 days:
That said, the opaque bottle your oil is in (and even the clear-ish strip on the side) will block most, if not all, of the UV light. If you're really worried, you can buy a 1L bottle of the same oil and compare it with whats in your 5L jug, but I don't think there's going to be an issue. My main worry would be if the jug wasn't properly sealed it would get water or bugs or something in it, but that doesn't sound like the case.