I had a Vega GT...stock motor lasted 10,000 miles. Sleeved the block, added a cam, sidedraft and some headers and won 4 years of SCCA autocross regionals. Sold it with 80K+ miles with nary any
oil being used between changes. Go figure.
Course, the rest of the car fell apart around it.
Porsche took GM's idea and built the 928 motor...did a better job and those motors did quite well over time...unsleeved.