Cultus engine was made in late 80's old technology that need ZDDP and modern engine not need ZDDP. Most of the modern engine oil like synthetic oil have very little amount or absent the ZINC content in it. CULTUS need it i my opinion using synthetic in cultus is waste of money. Go for good MINERAL BASE oil like formula Caltex...
The change interval of Caltex Formula recommended by Caltex Dealership is 5000km but I change after 4000km. I never touch to 5000km, max 4500km is good for engine and most importantly is fresh the air filter after every 1000-1200km which improve oil life and fuel consumption too...