If the previous owner followed the proper guidelines on coolant he would have added a number of "tablets" - supplied only by Holden with his last coolant change. These "tablets" are supposed to lubricate the water pump among other things and will leave a brown oily deposit on the radiator cap. Also a lot of mechanics will use grease or oil as a gasket/sealant when assembling engine components - this then finds it's way into the cooling system and as oil is lighter than water it floats to the highest point - the radiator cap.
I wouldn't worry too much - like I said before it's normal if the car has been serviced according to the factory guidelines. The heater on Commodores is fairly slow to warm as the heads are cast iron - takes at least 2km to start giving heat...you could change your thermostat which may be jammed open - also check the heater tap....make sure it fully opens when you turn the dial to hot.
Yeah, check your oil next change... if its really bad (milky) its gonna be either your intake manifold gasket or your heads.... id go IM first as they tend to go more often than the heads do... either way it has to come off should something be wrong!