Changed before proton persona only 1 pc near the extractor. Result very obvious after just changed but will gradually increase back after 20k+ km, when removes, it's covered by black carbon again like the old 1. Root cause could be dirty valves, intake manifold and catalytic convertor which very hard to be cleaned.
Honda car can try clean the egr valve to improves fuel mileage as it's easy to access.

Ya Liqui moly brand like rm40+ per botol quite expensive. Mr diy aeropak brand cheap only rm10+ but dun know effective or not never try before.