I have a friend that switched from Synthetic to regular because his car was leaking about a quart a week... after finally getting it fixed (i think he changed CCV, valve cover and something else) he went back to synthetic he didn't have any problem. But here is what he did: He used to change synthetic every 8-10k miles, when he switched to regular he changed every 4-5k miles. He drove about 12k miles on regular 5w30 before he got it fixed and switched back.

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2001 BMW X5 3.0 Sold
2008 VW jetta S
2006 Lexus Rx400h