Hi Alex,
I'm no expert when it comes to oil but some peoples ideas on this forum (not this thread) can be totally wrong. Remembering oil is a very complex item and in terms of lubrication, there are many tradeoffs from start up lubrication to hot lubrication, and the amount of heat the oil transfers around the car.
In reply to your question Alex the 10w-60 seems ok for Canberra all year round. The manual (for my car and I believe yours) says 7.5-35w oil. So a 5 or 10 weight oil when cold should be fine. The higher number (hot) can generally be as high as you want, if you are racing the 60w is when most people recommend as the oil gets very hot and as you probably know oil thins out the hotter it gets, daily driving 40 is fine as the oil never reaches really high temperatures.
I've been running Shell 5-40w but I will be using the Castrol 10-60w in Summer or for any track days.
I've never changed oil on an RB25 but it can't be as hard as the RB26. Just don't rev the engine to get the last bits of oil out :-)