I wrote Vavoline Tech line with the subject question and below is the response:

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Switching between synthetic and conventional oils is not a problem. You can switch back and forth as much as you would like, the oils are compatible.
Synthetic or synthetic blended products are not going to cause any damages. If you are already experiencing a leak, the synthetics could possibly make the leak worse, but the products will not cause leaks.
For our latest promotions, please give our rebates and promotions department a call and the associates can assist you in listing our latest offers.

>>>> ----------------------------
>>>> From: tbcaudill@ashland.com
>>>> Sent: Monday, April 28, 2008 5:46:21 PM
>>>> To: ValvTechLine@ashland.com
>>>> CC:
>>>> Subject: Site feedback from Valvoline.com
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