Thanks Pat, that supplies the information I needed. For background, the engine was made on 4 April 1977.

Here is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6jmvGVJ1DajI.pdf

Here is the operator's manual:
http://bsintek.basco.com/BriggsDocumentDisplay/lrvvA-wU7eL7hAxB91ObQ.pdf

Briggs instructions for type and amount of oil are given in the operator's manual. Essentially, you fill with oil by putting the engine on level ground and adding oil until the level reaches the brim of the filler hole. It should then be holding 600 ml of oil. You need to tighten the plastic filler plugs with something like a screwdriver across the two projecting pips: finger-tight will not seal the plug, and it will ooze oil. For the Australian climate you should use an automotive-rated (SF, SG etc) straight SAE30 oil, not a multigrade. If you use multigrade no harm will be done but the engine will burn far more oil than necessary.

It is important to change the engine oil regularly - there is no oil filter, so oil changes are the only way to prevent nasty black sludge building up in the engine.