Should stated it was in reference to conventional oil.
Synthetics are more heat tolerant. Which means less scorched particles, and thus less acid formed in the same time. I usually go 10k miles when using synthetics.
Toyota was trying to save money on manufacturing. I heard the blocks had fewer coolant channels. Whatever the reason, the oil got cooked.
The lesson is, when you have problems with the engine and oil is suspected, get the oil in the engine tested.
The HHR has GM's oil monitoring system. During the spring/summer it called for a change at 7500 miles. In fall it went 6700.
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