Why would a 1993 Subaru Loyale 1.8 liter with 160 thousand miles use half a quart oil a week?

Some engines will consume some oil (by the rings most likley). A quart in a thousand miles is cosidered normal by "new car dealer's ". Those motors were prone to leak also. So if no leaks I would suspect it going out tailpipe . converter will shield visible smoke. oil is sometimes cheaper to add than to do a major repair. just keep it full and you will be fine. 576,600 miles is the mileage I changed the motor on a 89 gl wagon. It burned a Qt between oil changes after 300,000. Try switching to 15w40 to help with oil burning. 15w40=576,600 miles