Brian,
As far as I know, the rainfall distribution is far more important than the quantum, assuming a reasonable season. If the rain fall is during the time at which the seed is forming and before hardening, the seed will be larger than "usual". If, then, there is a lack of rainfall during the time following seed hardening (which is the time of flesh cell development) there will be little oil in the fruit. The oil accumulates as the flesh develops (both by cell division = multiplication, and b y cell enlargement) so if there is not enough rain then, the oil yield will be poor.