Im with mattsv6 with regards to the service intervals. There is no way in the world that I would let a car go for 15k without changing the oil. Depending on how many kms you actually travel in a year, I would do an oil change every 6mnths regardless of kms travelled, otherwise every 5k-7.5k if this comes up sooner than 6mnths.
If you change the oil this often, then its not really just about the quality/longevity of the oil so much. If you use a main manufacturers oil eg Castrol, Mobil, Shell, Penrite, etc and viscosity anywhere between 5w-30 and 10w-40 then you should be fine. Semi-synthetic is fine to use if changing oil this regularly. Oil only averages cost of $30-$50 for semi-synthetic and then the filter about $16 retail at Holden. $40-$70 every 6mnths or so is really a minimal amount for the health of your engine