Well just like oils, the plot has thickened!
Prolube Monotec 40 has just been recommended to me as a good oil to use. Prolube make it in 5 litre and 20 litre containers. Does anyone have any experience with it?
Interestingly, Penrite's technical department recommended I use S.A.E. 30 4-stroke Mineral oil. They also mentioned that they make an S.A.E. 40 Monograde that is mainly sold for use in older diesel trucks, but which would be entirely suitable for 4-stroke petrol engines. Problem is it only comes in 20 L drums!
This is what the original manual says about oil specifications.
The recommended oil for this engine is SOUTHERN CROSS Engine Oil - DG Service S.A.E. 40. If Southern Cross Oil is not available, any other S.A.E. 40 Detergent Type Lubricating Oil, which is recommended by any reputable oil company for service conditions classified by the American Petroleum Institute as Service DG may be used. S.A.E. 40 oils for MS, DG, DM, or DS Service can also be used.
This article provides a good "translation" of the old API service standards to the more modern standards:
OIL CONSIDERATION TO LENGTHEN ENGI...ion Agricultural, Utah State University. 
