Worse case scenario would be valve damage.

BUT I did notice that annoying clicking sound when my oil level dipped 1/2 quart or so. The sound would disappear after adding the oil or if I did an oil change. Another thing that my mechanic advised was if you use an aftermarket oil filter it may not have valve in it to prevent the oil from pooling in the filter (therefore less in the engine). Either use a Mann filter or original Volvo filters.

You can also use a heavier weight oil such as 15W-40.

Good luck.