I noticed your question has gone unanswered so thought I'd give my opinion! If you want to use a modern oil with it's detergents and friction modifiers etc.. I would have thought a 40 weight would be fine. In which case no reason not to go to a 10W40, to benifit from easier starting from cold, and maybe even go down a battery size.
If you're planning on summer motoring +2hrs at a time, when the oil will get much hotter, then I might be tempted to remain with the '50'. Also if you're planning on competition or high rev use I'd also stick with the 50 for added protection. (With the exception being sprint/hill-climb as the oil never warms up).
In general modern oils 5W20 or 30 are great in modern engines with more accurately machined bearing surfaces, with cranks that are machined with barrel on the pins and with 4 valve style heads with tiny valves, using tiny lifts, and then having all that controlled by hydraulic lash adjusters and roller followers. Also most modern engines use a water style oil heat exchanger which helps prevent oil getting 10-15C hotter than the coolant.
There are a host of Zephyr forums out there with a lot of expertise in Australia and New Zealand, but I'm sure there are other resources closer to home.
