It depends on how long you expect not to use the engine. Castor gets a bit sticky after maybe 6 months but a bit of fuel soon frees it up. After maybe a couple of years it can get very gummy and can be hard to get out of the bearings and the con rod wrist pin end. That's for normal castor like Castrol M but I'm not sure about medicinal castor. If you're thinking about putting it away over winter (isn't that most of the year in Melbourne? )then a bit of castor in the fuel and running the engine completely dry of fuel is all you really need. If it still worries you then put some ATF in it.