I have a ‘98 v70 w/255k miles. It’s our “extra”car. Can’t sell it due to body rust, but it still runs well and I can see driving it locally until it dies a natural death. I drive it regularly but only put about 120 miles per month on it if that. I use synthetic oil and 5K intervals. At that rate it only needs oil changes every three years. Manual says change oil at six months or 5000 miles whichever comes first. How often must I change the oil if I am using quality synthetic?