Buy synth and change every 9 months or less if money is no object. Also have your engine flushed with cheap oil (mineral will do) and do it in intervals of a week (tipong lagyan mo ng mineral oil then run it for a week. change mineral ulit run it for another week, then flushing fluid and change to synth.) In this scenario money is no issue
Huwag mo paabutin ng one year. Check mo oil after six months and after nine months. I had my engine in an older car changed and after six months the Helix oil was in bad shape due to the stop go driving in the city so I had it flushed (cheaply kasi for me money matters.).
When you checked an engine which was flushed well, the oil appears clean even after a week of driving. Tipong parang bago pa rin. If it doesn't, lalo na pag mineral, then it would be a good idea to flush and replace. IMO lang po.