You can probably borrow a free leak-down tester from one of the auto part chains... it will likely tell you where the leak is... although if you are still low on compression after putting a little oil in cylinders, it won't matter much...the head has to come off.

I have seen only one instance where a piston or piston ring failure has caused the kind of compression loss you are seeing...It's almost always in the valves...or the head gasket..

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'99 E39 528i 5-speed 130k
'06 325Cic auto 115k
'05 X5 3.0 auto 173k-SOLD
'03 530i - manual swapped - 430k miles-SOLD