The 1999 Corolla with the 1ZZ-FE (non VVT-i for 1999) should be 8,000km or 6 months, whichever comes first. 97% of the oil in the market is designed for manufacturers specified change intervals, even most synthetics. Oil is changed by distance because additives build up and the oil gets dirty, but also by time because oil can absorb moisture, especially if it's run on short trips frequently where the engine has not had time to fully warm up.
Amsoil, Mobil 1, and a few others have oil designed for extended use up to 15,000 mile/1 year in severe service, combined with a long life oil filter which has about 3x the dirt capacity as a regular filter. The oil itself has more additives that protect the engine, maybe better moisture absorption resistance too.
It's hard to tell if this has caused any harm to the engine. If the price was right, I would take a chance. Otherwise, walk away.