I'm debating to switching to synthetic oil on a 2004 V70 that I purchased pre-owned from a private seller. The car has 17500 miles on it and had its last oil change at 15,000.

Any reasons out there for not switching? How many miles can I go between changes?
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Geo

By all means, switch.
How long you can go between drains depends on the oil. There is more difference between synthetics than there is between non-synthetics.

Any name brand synthetic should be able to go at least as long as the manufacturer's recommended drain interval. I've been using (and selling, so consider this biased) AMSOIL in my turbos for about 20 years now, and changing every 10K. And I know I'm throwing away good oil---

I'd switch her over to Amsol or Mobil 1 in a minute. Little question that the syns are superior to dino oil although there are many engines with millions of miles that have never seen anything but dino oil. Key with dino oil is changing it on schedule. With the syns, you'll get all kinds of opinions on changing. I change mine April and October.

bl 3 bricks with 400K and going strong

You didn't mention if your car is turbo or not. If it is a turbo you will be doing the car a favor by using synthetic. The turbo bearing(s) run very hot and mineral oil tends to "coke up" from the heat. Of course you will not have a problem with dino oil if you change it on a strict schedule-maybe every 3-4K at the most. The additional cost for syn oil is absorbed partially by extending oil change intervals to 5-7.5K You have to choose what is best for yourself but when I plunk down $35K or more for a luxury car I wouldn't scimp on the oil (lifeblood of the engine)