Toyota does not recommend synthetic oil in their manuals, but they do recommend 5W30. What puzzles me is that the dealership I was using would put in 10W30 instead of the 5W as recommended. Could my warranty be voided when the dealership uses a different grade of oil than what was recommended? I must point out that I live in the south where the thinner oil is not as critical as in the north, so that could be why they were using the 10W30. But I switched dealers (for other reasons) and they new ones are using the recommended oil.