The first conclusion many mechanics will arrive at is that the engine’s head gasket has blown, allowing the oil and coolant to mix. Have a look at the dipstick. If the engine oil is milky, then this is a very real possibility. Perhaps when the cylinder head was replaced, the gasket has not sealed properly.

But if the oil cooler was replaced because it was leaking into the radiator (which is pretty common) then you might find that the oil you’re still seeing is just residual oil from the previous failure, and not a problem at all. Even so, it would be wise to replace the coolant and flush the system to try to eradicate all traces of the old oil.

A workshop can perform a chemical-based test to see if the head gasket is leaking. It’s worth the relatively small cost to rule this out and begin to look elsewhere.

The car is still under warranty, so you should expect that the repairs would be covered. It’s no unreasonable that they want the car for a full day; you have to give them every chance to find the cause. As they claim they couldn’t find the problem when driving the car, it would be a good idea if you were to test-drive it with them to demonstrate the problem you are concerned about. That way there is no doubt in your mind or theirs about what it is you feel needs to be fixed. The fact that the transmission oil is very black suggests there is something wrong internally. Have the diagnostic check done, and then work out who pays for what and what needs to be done to get your car running again properly.

I wouldn’t recommend the Cruze, there have been too many problems with them. The Toyota Corolla, Mazda3 and Hyundai Elantra are all good, reliable cars with plenty of performance for the class. Any one of them would be a good choice.