Synthetic oil is not mandatory after prior use, but continuing it is recommended. Use a quality full-synthetic oil and a quality filter with a working anti-drainback valve, and change it about 1,500 miles beyond a normal interval or around 5,000 miles.
The owner of a 2000 Mitsubishi Eclipse GT with 94,000 miles asked whether synthetic oil must be continued after the previous owner used it, and how often it should be changed. Experts said switching to conventional oil is not proven harmful, but recommended staying with synthetic oil for better protection against sludge, heat, and wear. They suggested adding about 1,500 miles to a normal change interval, with one owner changing around 5,000 miles, and emphasized using a quality oil filter rather than a cheap filter.
