This article is from AutohausAZ:
Traditionally most engines were expected to burn a little oil and adding a quart every 1000 miles was considered normal. As emissions and fuel mileage demands increased throughout the late 20th century the demand for much more efficient engines became paramount. Tightening tolerances helped in both regards to burn fuel more efficiently which then also meant unburned fuel and a bit of oil were less likely to go out the tailpipe. By the turn of the century many engines needed very little oil added in between oil changes.