Aiven, try switching back to Perodua Semi Syn 5w-30 if possible (gold bottle, API SM) and then see hows the result. I personally do not think that it has anything to do with the gearbox for now, unless one possibility where the gearbox efficiency dropped when it heats up. Check the ATF also and change/flush if you feel that it is necessary or has the budget.
I've been using P2 engine oil (which is by Petronas) for 2 years and im quite happy so far. The only drawback is that the car starts to feel sluggish after like 6000 km. P2 service interval is 10000 km. But well which engine oil can lasts you for 10,00km and still feels like new at less than 100 bucks?