Hi Terry,
Well, I have been beaten to it, Clive has a very good article there on the Lubricant codes. To define it further for you, in relation to your Saracen, I do not recollect OX-8 being used in them. This is a DOT4 brake fluid and must not be used in this vehicle, what is used for hydraulic brakes and steering is OM-13 which is a mineral hydraulic fluid with the grade ISO 10, as the grades in the ISO range are very close in their viscosity, 12 or 15 may well be alright, given the ambient temperature in Australia. It might be worth checking any Australian army manuals about what engine oil grades they used in theirs, due to your temps being high than the UK. The gearbox does not need any fancy oils, a SAE 30 engine oil is used in them, with transfer box and all other gear oil fills (OEP-220) being a Hypoy or EP90, but ensure it is one that is compatible with "yellow metals", ie. bronze, brass, etc. XG-279, just an ordinary chassis grease and PX-7 is same as Vaseline or petroleum jelly available in drug stores.
One point regarding Saracen suspensions, where there are nipples on wishbones, etc. do not use grease in these, but use the OEP-220 equivalent gear oil that is safe with bronze.
Hope that has defined it a bit for you, if in any doubt please send me a PM.
regards, Richard