Why not consider Lion Electrics "ProGLow Synthetic Mix?" (Lion are in Perth guys)

It's a Castor/Synthetic blend and when mixed at a ratio of 20% oil, (And whatever nitro/methanol mix you chose) actually gives a 15% Synthetic--5 % Castor ratio.

I switched to it when Castrol M prices shot up to what I considered to be an unreasonable level for 1st pressing Castor.

You get the best of both worlds as far as the high temp protection of Castor and the internal cleanliness of Synthetic, as well as the anti rust protection of the latter for the engine bearings.

I am also enjoying not having to teardown my 2 stroke engines for cleaning so often, especially my ringed 2 strokes which were forever getting gunked rings which would then freeze. (More time for flying and saving $ on new rings)

I haven't had any problems as far as overheating in summer goes and my engines are cleaner inside and out.

Even a couple of Fox engines I have (Fox recommend 22% Castor in their engines, 'cause they run hotter than most other 2 strokes)
run on blended synthetic without any trouble.

Your LHS should be able to get 5 litres in for you.

Oh, as for 4 strokes, don't even conside using Castor in a 4 stroke unless you want to spend most of your spare time reassembling valve gear and timing the whole works.

Castor, including the much vaunted Castrol M, really messes up the insides of 4 strokes and does so very quickly.

As for Methanol, any of the major oil companies Methanol is ok.

As a former Oil Refinery worker (20 plus years) for one of the majors, I can tell you none of them have a methanol production plant in Oz.

They all buy it in from elsewhere and repackage it under their own names.