My mechanic - a sharp, intelligent and hard working guy stopped me from using high octane as i had laser irridium plugs in my honda city which are supposed to be economipon fuel whereas high octane is supposed to be consumed more for better perfomance. The car was giving me trouble and change in fuel sorted out my problem.
Since grande has dual vvti which i think again play its role to consume less fuel therefore i guess only high octane will not be suitable for this. Mix it with super.
Also, you need to check who is selling best super petrol in town.
Please note i am not an engineer nor an expert on engines. I just narrated my experience. You should also discuss the same with ur authorized toyota mechanics. Or let other pak wheels member to reply.