I'm going on vacation for 3 weeks and am due for an oil change right at that time. Where I live is cold, I may be able to store it in a garage however. My car is a 2017.
Should I get the oil change before my trip and let it sit with fresh oil or wait and get the oil change when I get back?
There is no correct answer to this. It is only a matter of preference that will give you peace of mind.
Since the car is not being driven, the oil change can happen at either point. You will see no added benefit either way.
While it sits for 3 weeks, the old oil will not become bad or damage anything. Changing it before you leave will just be one less thing you need to do when you return.
Change it after, especially if you are having the oil filter changed at the same time (which is often recommended.)
It is always possible that the mechanic will carry out the job incorrectly, and fail to replace the plug or filter properly. The filter gasket is quite large. The chances of a problem are low but I wouldn't leave a vehicle (or a house) alone after any event that could potentially cause a leak.
If you leave a car with an oil leak for 3 weeks you will have a garage full of oil, and a car that is undriveable. If you notice a leak between getting the change done and leaving on your trip, you will have to rush back to the garage to get it fixed.
If you do it after you come back, you should notice a leak immediately and be able to take corrective action without a sense of urgency. You will also find it easier to get a fix from the garage if you bring the faulty work back immediately rather than waiting 3 weeks.
According to How To Prep Your Car for Long-Term Storage:
Change the Oil
Skip this step if you're only storing the car for a week or two. Consider getting the oil changed if you will be storing the vehicle for longer than 30 days. Ford recommends this in its owner's manuals, saying that used engine oil has contaminants that could damage the engine.
So, since you're going to be gone for 21 days, you're still within the margin of not needing a change before you leave, but used oil does cause some damage when it just sits there, so fresh oil is probably a better idea overall. If you had just gotten an oil change, doing it again would have been pointless.
Since used oil is apparently a problem, I'd go with a change beforehand. If you get a synthetic oil change (costs more), you can actually go longer than the "recommended" 3,000 miles, which is actually a myth. Modern synthetics are good for six months or 8,000 miles between changes. You might also be interested in reading Three Months, 3,000 Miles Or Longer?: The Truth about Oil Changes.
If your changing the oil yourself, I would suggest to drain the oil, leave it with the drip pan underneath and drain plug removed whilst you are gone and when you come back, fill it up with oil again. This way more of the old oil will drain out. To ensure you do not forget what you have done, disconnect the battery and leave a note to yourself ontop of the battery. That way if you go to start the car with no oil, the car will not start and the note will remind you.
It is never necessary to leave an engine drain for so long but I thought it might be of some advantage and offer different perspective to other answers. Nothing bad can happen if you leave the engine with no oil in it. There will be enough of a film to prevent any rust (I did this for a number of weeks to maximise the amount of oil drained before removing a sump. I then left the sump off for some more time whilst the car was outside and no rust developed). If your car is stored on street parking I would not suggest this approach as the oil pan could become disturbed and result in spilt oil and your neighbours will not be happy.
If you do it after, it will be an additional 3 weeks before you have to do it again.
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I'm thinking before. Old oil has tiny particles which might pile up to bottom by the time and as you drain the oil out the tiny particles will not drain out as well.
Of course you can, and you should, run the engine for a good while before starting to drain oil out (also let it cool down a little) - still I would prefer before.
Would you leave your car with old oil for a longer time, say like a year? I wouldn't, I'd change the oil before.
