For years, I've only used Penrite when I service my own vehicles. Never given me any dramas at all, reasonably priced and is pretty much available everywhere. Checking their website, your car would apparently use their 5W-40 Enviro+, which at Super Cheap can be had for around $65 for 5 litres.
I don't own or have yet owned an Insignia, let alone a diesel, but I also have had a few diesel vans for work and have found the Penrite oils to be fairly good for vehicles that cop daily abuse. Although, as my current van is serviced by the dealer, they run a 5W-30 Caltex Havoline diesel oil through it and I don't seem to burn any off over the course of the 15k km between services either.