I have a Mercury 500 Blue Band, yes I know it is old,. but it runs very
well
and has never missed a beat.

I was originally told to use leaded fuel with 50:1 oil mix. The fuel in
Australia is now lead free. The next suggestion was that I use premium
unleaded and add a bit more oil.

There is no reason to run leaded fuel in a outboard 2-stroke engine.
Mercury's only concerned with the octane rating of the fuel. I would contact
a Merc dealer for the octane requirements of your outboard.