X5 E53 4.8is oil in coolant

[peter71]

Hi, does anyone have any idea what causing the problem:
Recently fitted a second hand engine by my mechanic, engine bought from a trusted seller, and after when 1st started with a new engine just after 1 minutes some oil appeared in the coolant expansion tank. not a big amount just the top of the water
Engine running well, it was driven for 15-20 minutes for testing, water temperature ok, coolant pipes not hard, no water in the oil, no smoke or sweet smell from exhaust but my mechanic recommended a head gasket replacement, but Im wants to make sure whats wrong with it, might be a cracked head or block, so I dont wants to do any unnecessary work
there is any more place where the coolant able to mixing with engine oil?
any hose or oil cooler, any joint?
Thank you in advance