Our neighbor's large pin oak tree provides beautiful shade for our cars and properties but for many years it has consistently dropped pin-point size drips of syrup onto our driveway (cars). It is really quite a mess. The former owner, now deceased, said oxalic acid sprinkled on the ground would reduce the issue. Is this valid and, if so, is it safe?

Dripping sap is usually the result of insect feeding within the canopy. This can be from aphids or scale crawlers. Both insects have piercing/sucking mouthparts and any sap removed from the tree passes through their bodies, dripping onto foliage below. Aphid populations come and go, while scale insects can be a perennial pest with overwintering adults giving birth to crawlers which move to younger tissue to feed. Aphid colonies will have adults and young alike together feeding and in great numbers. This year scale insects have been very heavy this growing season. Once you determine which insect is causing the dripping, you should contact your local extension office for appropriate treatment options for Iowa. I am not familiar with oxalic acid and cannot comment.