Previously opened bottle of oil - is it any good?

[Dr Mike Oxgreen]

In a nutshell, my question is: to what extent does modern fully-synthetic oil degrade after the bottle has been opened?

I bought a 5 litre bottle of Castrol Edge 5w30LL about 18 months ago. I opened the bottle and started using it for top-ups, keeping it in my garage, in the dark, with the top fitted. It now has about 3.5 litres remaining.

I’m planning to do an extra interim oil change on my 2012 Skoda Superb 3.6 VR6, for which this is the correct oil and a change requires about 5.5 litres. These days I do a somewhat higher proportion of shorter journeys, so extra oil changes must be a benefit - forty quid’s worth of oil is half the cost of a tank of super unleaded, so to me it’s a no-brainer.

So which do you think I should do:

1. Buy a 5 litre bottle of oil, use all of that, and use a little bit of the “open” oil to bring it up to the right level, keeping the rest of the open oil for ongoing top-ups,
2. Buy a 4 litre bottle, use all of it plus a bit more of the “open” oil, keeping the rest for top-ups,
3. Use all the “open” oil, and supplement with half of a new 4 litre bottle, keeping the rest of that for top-ups.

If you think the open oil is absolutely fine, then option 3 makes sense. But if you think it’ll be seriously degraded then option 1 is better.

What do you reckon?

Edited by Dr Mike Oxgreen on Tuesday 6th January 17:25

[2 sMoKiN bArReLs]

I'm no expert but..

...I had a huge (think gallons) of Mobil 1 in a drum in my garage. It lasted years & I topped my TVRs up from it over several years with no catastrophes.

But, all the cars burnt 1 litre every 1,000 miles, so the oil never got old

Isn't oil cheap enough to just buy some more?

[2 sMoKiN bArReLs]

I meant option 1

[paul_c123]

Being open or still sealed isn't the issue, but they do degrade over time (even if sealed). Also, being outside in the cold, eg in a shed, is bad for oil. And you should shake the bottle before pouring it into the engine.

[RacingStripes]

Option 3 every day of the week. It'll be fine.

[Opapayer]

Fully synthetic oil in an unopened steel container should last for decades. In a plastic container probably 5-10 years, as the plastic will gradually degrade. An opened bottle of fully synthetic should be good for 5 years from opening, as long as you’ve kept it sealed when not using it. Plenty of cars are 2 year service cycles and that’s for oil that’s actually being used, possibly daily.

I’d use it up first though, as it’s been opened. For reference I used to have a huge tub of semi synthetic that I used for motorbike oil changes that used to last about two races seasons from opening to empty and never had any oil related engine issues.

[ridds]

As others, it'll be absolutely fine.

I deal with 1000L IBCs of engine oil and all we do is stir them with the decanting pump before use, as you can get separation of constituents.

We also leave the IBCs outside, as long as they are covered to protect from water ingress and UV then no issue.

[Dr Mike Oxgreen]

Cool… I think I’m leaning towards using up the opened oil and supplementing with new. Lots of sources do mention giving it a shake to redistribute the additives, which seems like a good plan.