From the completely un-educated here, it may just be sludge that builds up over a number of years/miles.
My V6 is about to roll 200K, when I had my cooling woes last year got the milkshake gunk in the rad cap.
Cleaned it up and haven't seem it since.
My oil is changed every 7000K, the coolant was swapped out at 140km when I got the car and 3-4 times more because of my cooling woes.
Not much more here to help other than....that's life owning a G8, clean it up and keep an eye on it.
I agree, I mean if there is no evidence of coolant getting into the engine block, and the engine is not overheating. (Or sounding like a running diesel.) Then I'd just change the oil and coolant (along with the oil filter) while examining everything for any abnormalities.
Probably the 100% way to determine if it is really engine oil in the radiator. If the brown stuff turns up as globs (think lava lamp) after you collect the fluid then its engine oil. If its just brown colored, then that is just dirty coolant.
The worrying part is the oil pressure/low level warning. If that came up but the engine is running fine and oil & Cooler levels are normal, (no weird behavior from the car either) then it might actually be a bad oil pressure sensor giving you a false positive.
But I'd definitely change the fluids first and examine for any signs of leaks/cracks, to rule that out. After wards you can take it for a test drive to see if there are any changes in behavior. Aside from car acting a bit more 'awake' from having fresh coolant put in.
As for oil filter housing, there are a few videos on that on YouTube by Aussie mechanics, I don't remember the title but your car trim is called Holden Commodore VE SV6 in Aussie-land so searching for that may help.