I found more info about MAHLE on a Volvo Forum:
"The collar gear research has been occupying most of my time lately, but I have made some progress on the oil filter shootout.
We tried to do some flow rate tests, but honestly the differences were so slight I could not get any meaningful data from our rudimentary rig.
Here is the test chamber:
We conducted the test with mineral oil as it is just easier to deal with and pump. Unfortunately the pump we used could not generate any meaningful pressures for us, I can say however that the STP/Puroilator filter was the only filter that showed a noticeable backpressure. These pictures show a 100psi gauge, which was replaced with a 0-5 psi gauge for actual testing.
As you can see the cores are all very similar with the exception of the Volvo/Mahle filter, here is a closeup:
And all together:
The ridges on the Volvo/Mahle core prevent the filter from fully collapsing even under pressure and help keep the pleats open under pressure, this is absolutly critical to filter performance and the design is rather functional and elegant. None of the other filters feature this design.
Filter surface area ended up as follows:
Fram CH8712 - 242.0 sq in
Mann HU819X - 232.0 sq in
Mahle OX149D - 219.9 sq in
Volvo 1275810 - 232.5 sq in
Wix 57021 - 253 sq in
K&N HP-7002 - 228.4 sq in
AC Delco 2250G - 229.3 sq in
Bosch 72206 - 229.3 sq in
STP/Purolator - 189 sq in
The highest surface area was provided by the WIX filter, followed by the Mann filters and then the Mahle/Volvo filters
Overall here are the conclusions:
Best filter to use:
Volvo/Mahle. The filter surface area might not be as high as the Mann filter, but the crush resistance design and the better functioning seal make this the best filter to use.
2nd place: WIX With the highest surface area, and excellent filter media this filter is only missing a core designed to keep the pleats open under pressure. The seal works well despite it being designed differently from the Volvo OEM filter.
3rd place: AC Delco / BOSCH These filters have good filter media, a sturdy core design and good surface area. The seal arrangement is similar to the WIX but the corrugation on the filter media is very basic and shallow, it is less likely to perform as well under pressure.
4th place: Mann/K&N/FRAM The production variations on these filters is a problem. Pleats were uneven and when cutting the filter element apart we noticed that the filter media was not attached well to the end caps in some places, also, despite the fact that Mann made all three filters the production variation between them was quite marked which is not a positive thing. The seal mechanisim is really the worst in the bunch consisting of a fuzzy fabric ring on the top and bottom of the end cap.
5th place: STP/Purolator What can I say? Anyone who buys this filter and knowingly puts it in their Volvo should have their car taken away from them. This single layer filter is clearly the worst i the bunch, it was the only one to show restriction in my tests, the pleats have no corrugation to prevent collapse under pressure, the pleats are horribly uneven, the surface area was the lowest of the bunch AND it was possibly the most expensive filter I was able to find. If you have no other choice but the STP filter when you change your oil, I'd just leave the Volvo filter in for another 7500 miles than put a new STP filter in... "
Operating Pressure Max) 900 kPA 1200kPA 
