Have owned this car since new and have always run Penrite 5w30 full synthetic oil in it but have always been a little disappointed in the fuel economy.
I change the oil & filter every 7000 k and the car has the usual mods, including a remap and 3 inch exhaust. It tares in at 2860 kg so she is heavy. Using the old oil I used to always average 13.8 litres per 100 kilometres.
Now I have gone to this new Castrol A5 oil as it states, it is recommended for rangers and mustangs, my fuel economy is back to 11.2 litres per 100 kilometres. Very happy with those figures.

I've been using Castrol Edge Titainium for a number of years in all my cars. I have tried most of the readily available oils from supercheap auto. After a recommendation to try Castrol Edge in my VS SS commodore I was extremely impressed with it's performance. I change every 10,000km and it is clean. I believe it could easily do 15,000km.

A tip for people wanting to try it but are scared of the high price. "Stock up when there is a sale". Best time of year is around the Bathurst 1000 race.