I'll second what Noodles said, but I choose heavier weight oil like 20W-50 or heavier to top off used oil. Reason being they are better for the engine and help recharge the old oil that has broken down. So to make it simple I'd throw in a quart of common Mobile 1 15W-50 and call it good.
Also, you have no idea if the seller reset the oil timer without changing the oil. A used car seller with a Benz that has an oil change light will do what? Spend the $ and time to change the oil with top quality oil, or just reset the timer? So imo I'd change the oil altogether.
Optionally you can read my much longer caffeine fueled reply below
You can also mix regular oil in it too, doesn't have to be synthetic. I like to use thicker oil to top off what has been lost and usually you can't find thick synthetic. Come oil change time I'll $pend the extra for synthetic on a nicer car, otherwise I'll just use mostly regular oil and only 1-2qts synthetic. Note they now sell oil that is a mix of regular and syn, but it's way more expensive than if you just mix it yourself.
10W is the lowest I'd ever consider using but I almost always use 20W or more. I live in the desert so the temp in the morning is usually 50-70F. The first # (10W or 20W for example) # is simply how thick the oil really is, and 20W is better for the engine overall and not too thick where I live. If you live where it's cold, like it snows, then 20W may be a bit thick because the drawback to thicker oil is all oils gets thicker as it gets colder. So when you first start an engine that has been sitting for hours it will have drained some, maybe most, of the oil back down into the bottom of the engine where it's not doing you any good. When you start it the engine drives the oil pump which has to suck the thicker cold oil from the bottom and up into the engine where it's needed. This normally only takes a couple seconds but the colder/thicker the oil is the longer that takes. They say these few seconds at start up is where most of the wear on a typical engine happens.
Picture a fresh Vanilla shake from Jack in the Box and you're drinking thru a straw. Not so easy and you don't get much, but if you let it warm up it's much easier to drink. Oil is not nearly that bad but it's similar, so when it's really cold the thicker oil takes longer to get where it needs to go, and the colder it is the longer it takes. This is why people use thinner oil where it's cold. So 0W will flow much easier/quicker when it's 50 below out that 20W. The biggest problem, imo, is while 0W flows better to get oil to the engine sooner, it's not as good at protecting the parts for the entire rest of the time the engine is running. Also, when you turn the engine off the 0W will more quickly run out of all the places and passages the oil is supposed to be. The thicker oil takes longer to drain out and more of it remains on the parts, plus the thicker oil protects parts better. So when the engine sits over night with thicker oil it's not as unprotected as most people believe. So imo I would only use thin oil if it was so frikkin cold out that it literally takes too long to pump. How long is too long? I don't know, and hard to answer because it varies drastically depending on the engine design. For example I spent a winter where it was below 30F every night and often <0F but I kept using my 20W-50 oil and it was just fine. I could hear the oil taking longer to get where it goes, but I believe the better protection of the heavier oil is well worth the trade off of it taking a couple seconds longer. At some point it'll be cold enough that 20W is simply too slow and it'll be worse for the engine, but at what point that is will vary drastically depending on the engine and circumstances so you just have to make a guess. Or you can listen to the mfg of the eng and/or oil and go with the thinner stuff all the time. If I thought my 20W was too thick when cold I would simply use a heater to keep it warm. Many people use block heaters to prevent the water from freezing, but it also fixes the oil problem. So with the heater in mind for extreme conditions I see no bennie to using thin oil. Or you can look at it like this; people use thinner oil because they are unwilling to too lazy to use a heater.
The numbers are confusing, like wth does 20W-50 mean? The first # is what the oil really is, so 20W-50 is really 20W oil, which is how thick it is when cold and the higher the # the thicker it is. The second # is how thick it is at full operating temperature, which I believe they measure at 200F, 212 or somewhere in there. In the old days 30W was just that, 30W when cold and 30W when hot. Yes it's thinner when hot, but they're saying 30W oil is this thick when cold and this thick when hot so it's a reference point for oils today, which are different. So if you have 20W-50 it's really 20W oil and is as thick as 20W oil should be when cold, but it's newer tech so it doesn't get as thin when it gets hotter so it acts like old timey 50W when hot. I say "acts like" because it is not as good as real 50W oil, but is as thick. It's like comparing real Maple syrup to the fake stuff, they're as thick but they are NOT the same thing
So 20W-50 is still just 20W oil, but it beats 0W any day of the week when it comes to actual protection of engine parts.
Another note; thicker oil is harder pump all the time and it drags the engine down a bit since all the parts are in a thicker sticker goo of oil so your mpg and power will suffer a bit. This is imo why mfg's push the 0W and 5W oils, they want to meet the strict emission and mpg requirements. All they care about is the engine surviving past the warranty, which they will do on 0W but for how much longer past that? The mpg losses of thicker oil is generally not noticeable to the avg driver but the added protection of thicker oil is something the engine will notice over time. Unless of course it's really cold where you are and thick oil starves the engine too long when starting. If people are worried about the teeny mpg loss they can counter that 1000x over by planning trips better and ordering more stuff online instead of driving.
When I mentioned adding thicker oil to older oil I mean that oil gets thinner with use, and its lubricating properties also break down. So if you had 10W-30 in it but now have 10K miles on it, it is no longer 10W but something substantially thinner. How much thinner depends on how hard your engine is on the oil, but this is why I use thicker oil to top off old oil. Lets say your 10W is now 0W, if I add a quart of 20W I'm thickening it up a bit which isn't a lot but better than adding 0W. I've also used 60W but usually I use oil that is much much thicker to really boost the used oil.
Fyi; odds are that most of the missing quart of oil went into the intake system via the PCV system and intake valve guides, both of which end up on the valves. In short this is bad for the engine because it puts burnt oil deposits on the valves which hurt mpg and emissions. Valve deposits are what you see in commercials for fancy fuel additives like Techron or whatever that's supposed to clean that junk off. Well, they're not all that good at cleaning it off so it's best to prevent the oil from getting in there in the first place. That's a more complicated fix but thicker oil is less likely to end up going through the PVC system or guides so another point for thicker. The 0W may be better for mpg and emissions at first, but over time an eng running thicker will have lower emissions and better mpg due to being cleaner and having less wear. Imo