Hi head gaskets do not always put water in to the oil it is more the other way where gas is introduced in to the cooling system and via the exhaust gas escape water can get back the other way in to a cylinder, ask the garage to do a sniff test on the header tank, this involves !Removed! an adapter with a reservoir of the testing fluid to the expansion tank then run idle for 20 mins, the fluid will change colour if it detects exhaust gas, I agree with Steve a full scan is your best route.
Steve.