Just close the loop on this...I was changing my filter last night and ran into the exact same issue (I got the purelator) and the bmw place I had it changed at last had used a Mahle....after putting in the purelator, it was leaking out of the oil filter....I checked the two against each other for size. Both filters are the same size, however, the black rubber rings on both sides of the filter (the ones that are about a US quarter in size./..not the O ring) on the purelator, they're slightly bigger than the mahles (maybe 1/16th of an inch)...just enough to not make it seal up properly causing it to leak out of the filter....at any rate, just wanted to throw that out there...
Jay