<p>For a new 4 cylinder Corolla being driven in the Bay area, 5-20 weight oil is fine for 5000 mile oil change intervals. 5-20 weight oils are semi-synthetic (80% regular oil/20% synthetic). The synthetic component helps keep the oil “in grade”. Your engine’s oil galleys were designed for 5-20 weight to help improve gas mileage. Heavier weights would only reduce mileage. Lighter weights (0-20) would be fine if you lived in a colder climate. I doubt the Bay area and your car suffers from cold starts at -30. Of course, always change your oil filter when you change your oil. </p>
<p>To maintain your warranty: find a good garage and keep the receipts showing the date, mileage and grade of oil used. The quality of quick change oil places varies all over the map. I have a garage and can change my oil with no problems. This may not be your situation. Find a local garage or a quick change place that’s been around for a while. Talk to the manager to get a feel for the place. </p>
<p>The Toyota V-6 engines from 1999-2003 or so had problems with sludge buildup. The engine’s narrower oil passages would allow the engine to develop sludge if oil changes were not performed at recommended intervals. Consumers Report had an article on this some years ago and suggested using synthetic oil which handled heat and had better flow characteristics than regular motor oil. </p>
<p>Used motor oil can actually be filtered and reused. Usually it’s burned commercially to provide heat.</p>