It's true that there is a filter in the reservoir and the whole thing is not that expensive. If you have oil coming out of the cap it is probably due to foaming. The expensive power steering oil is better for not foaming and quieter especially in the cold weather so if you are using ATF then you might want to change that.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)