Most likely problem is the oil cooler just above the oil filter. You can do a sort of test on it without any pressuretesting equipment. Take it off the car, leaving the coolant hoses on. It comes off easily, there's just one nut screwed onto the same thread as the oil filter goes onto. Then prop it up with both hoses facing upwards. Carefully fill up the coolant side of the heat exchanger until the lower of the 2 hoses overflows then leave it for an hour or so. If the level has gone down it's obviously leaking.
I'm not saying this will pick up leaks which only leak under pressure but this was how I found mine was leaking.
Best of luck.
