10W/40 semi-synthetic, dont use magnatec, it's rubbish.
I found fully synth too thin, leaks past the seals and causes blue exhaust smoke, waste of money on older cars.
Try your Vauxhall dealer parts dept, they were cheaper than Halfords for oil and filter when I did mine. Try motor factors too.
Make sure you have the correct size torx key to remove the sump drain plug. It will be tight, if you use the wrong key you will round it off.
fred
November 2009