My old Yanmar SB12 recommends 30/40 oil but I am finding it difficult to obtain (in New Zealand). Any suggestions of an appropriate multigrade oil to use please.
Not sure what is meant by '30/40' oil. Lubricants specified for old engines were often monogrades, so either SAE 30 or SAE 40, dependent upon ambient temperature. In the vast majority of cases if they were able to issue the specification today it would be for a multigrade 15w-40. Buy the lowest API spec possible, API CF-4 probably the lowest in common use but API CD if it is available. Lots of info on the subject on my website.
Monograde 30 and 40 is available if that is the recommendation. The best thing to do, to find local availability, is post the question on a specialist old car or bike website.
Or you could chat to any group of ole bikers.
Warning - the choice of oil is just as contentious in these circles, a lot will swear by using mineral 20/50, a lot will say other stuff, the choice is yours. At least you will find out what the options are in your patch.