Current mileage is about 160,600 for a 1999 Corolla, the last oil change was in May at about 155,800 miles, oil was 5W30 Synpower with a NAPA gold filter. Have added 3 quarts of top-off oil during this oil change period- all synthetic. Running close to 1 quart down now, should I add a quart which would take the car out to about 6,000 miles for this oil change period or change the oil now. I understand the 1zzfe is tough on oil and interval for this car is 5,000 miles, anyone exceed 5,000 mile oci with this motor? I'm also debating whether to keep synthetic for the next oil change or go with a cheaper conventional especially for top-offs. Thanks