I just had to replace a fel-pro valve cover gasket on my MINI (1/2 scale X5 :-) ). It split in less than 1000 miles. If you aren't using OEM rubber, I would try that, too.

P.s. I don't see a problem snugging it up as long as you watch the torque values, but if the rubber has split, it won't help.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles