I use the slightly heavier semi synthetic because my engine tends to burn oil a little. I lose about a liter every three months. It’s perfectly normal for this type of engine and well within the Vauxhalls quoted acceptable levels. If you look in your manual at the back in the charts it is something like 0.5 liters of oil every 1000 miles. I know not everyone’s does this and it does sound alarming, some people say there’s uses nothing, its all down to the manufacturing tolerances. For this reason I stick to semi, as the thinner fully synthetic burns off even quicker, and if you forget to check it and it gets a little low, it might cause more trouble than the benefits of using it, whatever they may be?