Leave single-pin spares. Pick up your single-pin spares. Bowl cross-alley at your spares. They put oil on the lanes, to keep bowlers from wearing grooves in the lanes.
Oil is spread between the foul line and several feet beyond the foul line.
Most of the oil in in the middle, less on the outside. Someone else is throwing over the first arrow. If your ball rolls on oil, it goes relatively straight. If it rolls on a dry lane, it curves. The secret is to roll your ball so that if it goes to the left, it is in the oil, and will go straight into the pocket.