Autobarn have 5 litres of Castrol GTX 20W50 Engine Oil for $12.99 again.
Please note this is suitable for engines PRE 1990. I believe Autobarn still give you 10% off oil filters when you purchase oil (correct me if I'm wrong), so IMO this is a pretty good bargain for all the old Falcon/Commodore owners out there :)
Blurb taken from Castrol website:
Castrol GTX is made from premium quality base oil and additives. It protects against the everyday problems that your engine faces, such as high temperature wear.
Benefits
• Extends the life of your engine
• Provide better high temperature wear protection
• Superior protection against viscosity and thermal breakdown
• Keeps engine cool and clean
Applications
Castrol GTX is suitable for all petrol and diesel vehicles where API SG/CD or earlier specifications are recommended.
Performance Levels
• Exceeds latest API SG/CD specification
• SAE 20W50
Pre 1990? I've been using this stuff in my 2000 falcon with no problems, and i know plenty of others do the same.