I have an Astra G fitted with the 1.6 8V X16SZR engine.
I check the engine oil regularly, and have noticed that there is a sticky greasy substance appearing on the oil filler cap. Having spoken with the local dealer, they tell me that this can be condensation, which is emulsifying the oil. I've cleaned it out around the filler hole. (there were a good few finger fulls), and I will have the rocker cover off when it's next serviced to see if there is anymore in the top end.
Is this condensation? Is there a breather pipe to the rocker cover that could be blocked?
I've checked the oil in the sump, and whilst it is due for a change there is no evidence of water in the oil, and the same goes for the water; there is no evidence of oil. The engine runs as sweet as a nut, and there is no chattering to indicate any lack of oil at the cam, followers etc.
I'm not losing water from the cooling system, and the heater works spot on.
Does anyone have any ideas?