I have a 2015 S60 2.0L T5 with 108,000 miles (I'm the original owner) and the last 2 times I've had the oil changed, when I reach 6,000 miles I get a message to add a quart of oil. I'm bringing it in this week for an oil change but since I told them last time that the oil had run low and they didn't seem concerned I thought I'd ask here.

 

If you are getting 5000+ miles before adding a quart, while not perfect, you are doing just fine. I'd be concerned if you were going say 1000 or 2000 miles before adding a quart. My suggestion would be to check the oil regularly (say every 3 or 4 tanks of gas) and just plan on oil changes at 5K miles (nice round number) or add that quart and go for the factory interval. Also make sure you use an oil that meets the specs in the owners manual (ie full synthetic and the proper weight).