Go to a Vauxhall dealer spares department, they will sell you the correct oil for your car. Buy a filter at the same time.
The price is about the same as a factor, halfords etc.
If you are changing the spark plugs get them at Vauxhall too, the car will run better with them than with any other.
You can get cheaper parts from supermarkets, but is it worth saving a couple of pounds on something that will be in the engine for the next 12 thousand miles?
dav, October 2010