How well do high mileage oils work? My grandmother has a 2001 Chevy Prizm with about 125k miles. It has a problem with burning oil which I understand is fairly common on this engine. It uses a quart every 1k or maybe a little more. This has finally plugged the catalytic converter which she is having replaced. Since the only permanent fix is expensive engine work that exceeds the car value, she intends to replace the converter and just soldier on checking and adding oil as needed. Would a high mileage oil help her to reduce consumption or is moving to a heavier oil weight better?