I believe Quantum lubricants are rebranded or repackaged products, most likely from a major lubricant manufacturer (probably Castrol), and largely (but not exclusively) aimed at VW dealers, garages and workshops in the UK market. Synta is just one of the labels Quantum use.
Their products have received approvals for all current VW engine oil standards (stands to reason really), so it's just a matter of picking the right product for your Mk5 Golf (i.e. VW 502.00 or 504.00).
However, it is unlikely you will find Quantum products being distributed in Australia - though happy to be proven otherwise.
In regards to oil choice, as long as it has been approved to VW 504.00/507.00, I'd settle for whatever is cheapest or easiest to obtain, but that's just me.
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