Your oil light behavior sounds normal to me. I wouldn't be concerned about running over on the mileage for your weekend trip, even though, in my opinion, mileage is more important than time on your oil. If the oil level isn't down to the bottom of the two lines on the stick, don't add a full quart, it is not good to overfill. Anywhere between the two lines is fine, but not above the top or below the bottom.
I have two 960 wagons, a 1995 and a 1997, basically the same cars as your V90, with similar mileage. I switched to synthetic oil on both cars not long ago and have been very happy. The synthetic seems to do a much better job of keeping the tiny little intake and exhaust valves lubricated on these cars. I increased my change interval from 3000 to 5000 miles due to the superior lubrication of synthetic, so don't consider it to be more expensive. I transitioned to the synthetic by doing a change to synthetic blend for one oil change, and then full synthetic. I also think that cleaning the breather system, including changing the breather box is a good idea at this mileage.
John