If it's had Vauxhall dealer servicing it will probably be 5W/30 fully synthetic longlife Vauxhall oil as here http://tinyurl.com/36sa4j4 This stuff is very good and cheaper than Castrol etc as they sell it in 5 litre cans not 4 litres. Even cheaper if you buy off eBay in bulk. You can go to Halfords and buy specific diesel engine oil which supposedly has more detergent in it but I bet the VX dealers just use one grade of oil and charge you about £50 for 4 litres at service time!

Until recently Vauxhall had a page on their website that told you what oil they recommended, and now they don't.

This should tell you something, but what, I dunno.

Either they don't know themselves, or have been having so many oil-related catastrophes they have given up.

Or maybe they just want your money, they won't tell you point blank, you have to take it to a garage, they take all your money.

The question is - fully synth 5/40 or fully synth 5/30, which will prove to have worked best after 200,000 or 300,000 miles.

Obviously you need to change it before whatever they recommend. Halving seems to be a good rule of thumb, but dividing by a quarter is probably better.