Penrite also has a few 70s:
40-70
https://penriteoil.com.au/products/hpr-50-40-70-mineral
30-70
https://penriteoil.com.au/products/stops-oil-burning-30-70
25W-70
https://penriteoil.com.au/products/hpr-40-25w-70-mineral
However, there are several C3 oils which meet the fuel economy requirements of C2. These are ‘dual-rated’ C2/C3 oils. They have the 3.5+ HTHS requirement for C3 but demonstrate the fuel economy requirements of C2.
You can buy it straight from the source https://shoplubricants.vivaenergy.com.au/shop/products/passenger-car-engine-oils/shell-helix-ultra-5w40-sp-5l-910281819
Or from Repco...
I would run either something like ESP 5W-30 or Valvoline Restore and Protect 5W-30. You don’t want a xW-20 in an engine with historical timing chain issues in such a big heavy car.
With direct injection, you can also inject fuel during the exhaust stroke. That’s how diesels perform DPF regeneration (except those with a dedicated injector in the exhaust stream).