VW504 oil is 5W30 if it says different viscosity, then it most likely isn't VW504 approved oil. I use in petrol engines Interject injector cleaner (415Y) every 3rd fill up.
It depends where you drive and how but 7,500km max. is what I do on oil in engine.
Even the Supercheap Auto is selling Shell Helix UltraExtra for less than $75/5L now, so it is not that expensive to change the oil, especially if you can DIY.
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