Seems like you might have two problems here. The first is that I suspect you’ve may have fried the computer in the Holden in the process of jump-starting your other car. Jump-starting a modern car with on-board computers is something that should only be attempted by somebody who is a specialist. There’s a specific technique that goes beyond simply hooking up the jump-cables and hoping for the best. It’s all too easy to send a voltage spike through the car and send the computers into melt-down. That would explain why the car will not restart.
As for the milky oil and the sludge under the oil-filler capo, that sounds unrelated but still serious. A blown head gasket can lead to this exact set of symptoms and it occurs when the engine’s coolant is allowed to mix with the oil (and vice-versa). Sometimes in these colder months (particularly in Tassie) it’s not uncommon to get a small amount of frothy, grey muck under the oil-filler cap, but not a huge amount. And if the oil on the dispstick is also covered in oil contaminated with coolant, then you’re looking at a new head gasket at a minimum.
I’m not sure I agree that all modern Holdens were problematic, but the Epica, which was really a Daewoo (it was built in South Korea) with Holden badges, is definitely on the suspect-reliability list.
