I posted a photo of what I drain out of the catch can every 6 weeks or so. If there was no catch can that 4 ounces of oil and water would be sucked into the cylinders and burned with gas. Oil mixed with gas lowers the octane and can lead to misfires. The knock sensor would detect the misfires and reduce timing. The timing reduction leads to less fuel economy. Also that oil and water mix would leave deposits on the top of the piston and exhaust valve. Those deposits also can cause misfires.
The catch can is not necessary for the application, but for $100 I think it is worth it.

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2005 X5 3.0I