I also contacted Redline Australia to clarify which oil they recommend. They actually sound like they know what they were talking about, as opposed to the service departments I spoke to earlier. They do not recommend 75w85 for the JGC. It is too light for the JGC and does not provide sufficient protection in their opinion. They stated that manufacturers tend to specify the lightest grade oil they can as it will marginally improve fuel consumption figures. While the difference is not noticeable in the real world, the better fuel figures look better in the published vehicle specs.
For Australian conditions they recommend 75w90 at a minimum, with either 75w140 or 80w140 as their pick. The first figure is largely irrelevant and only makes a difference at sub-zero operating temperatures. The 140 grade provides better lubrication at higher temperatures as experienced in Australia, or heavy duty applications such as towing or off-roading. They recommend 140 grade for both diffs.
They also mentioned that they recommend 5w40 engine oil for the CRD but advised to use 5w30 as specified in the handbook to comply during the warranty period, even though they stated 5w30 is too light for the vehicle.
As an aside, they asked me where I purchased my Redline gear oil from and advised that they provide no manufacturer warranty for any products bought from GL Lubricants who are a grey importer. (I suspect the same holds true for Mobil, Royal Purple etc) GL Lubricants however, claim they have no liability for any product bought from them according to their website. I checked and at the foot of the homepage in 6pt grey type that is barely noticeable is this disclaimer:
Disclaimer: We, as GL Lubricants Pty Ltd, will not be held liable for any manufacturing defects or non-performance issues of the products we sell.
Something worth considering when purchasing from them and contrary to Australian Consumer Law.