Mayo under oil cap common with Astra 1.6 8v's (I own one). Don't worry about it, caused by short journeys. If your temp gauge gets up to heat (half way) on temp gauge. Then all ok. If temp stays cold all the time. You'll need new thermostat. Have to take cambelt off to change it. Good time to change cambelt kit.
If not loosing coolant, then won't be HG. Try changing your plugs first. If not plugs,, then 9/10 coil pack. Buy one from breakers for around £25 (off car). Easy diy change, need to take battery off to gain access. Coil pack held on with torx heads.
This forum will help you:- http://www.vauxhallownersnetwork.co.uk/forumdisplay.php?f=83
George (Boroboy)
March 2009