Rumour (no theory!) has it that most olives are not self-fertile and therefore require "pollinators". This is normally just another variety olive tree. I initially thought it would be nice to have a couple of different varieties - just to keep my options open - not having all your chooks in one pen. But by pure accident (or luck!) this is generally seen as good practice. Most researchers are now recommending that growers plant at least three olive varieties in close proximity in their groves to ensure some cross-pollination to improve fruit set.
According to one source some varieties are self-fertile (my understanding was that Manzanillo falls in this category, but have since read that they also like to "pair", namely with Sevillano). Other varieties are self sterile and most are somewhat self incompatible. Pollination can take place over a 40 metre distance, by means of wind, birds, bees etc.
Specific varieties are incompatible (!) - these include Barouni and Sevillano. Why don't they like each other? Another is Manzanillo and Mission. So Mission is out of the question, not that I've seen any commercially available in NZ. And Barouni as well, if we can find some Sevillano to help pollinate the Manzanillo.
If you want table/pickling cultivars producing trees, stick to table/pickling for cross pollination and leave the oil cultivars out of the equation and vice a versa.
All this means that we are re-thinking our options. The million dollar question still beckons.