I used to have a COE car, I owned it for 15 years, 1st owner, in perfect condition after complete overhaul which cost me $4K.
Then I itchy finger started adding additives (fuel saving tablets from Nature's Glory), at first the fuel consumption dropped a little (AND with increased power too), but after several tankful of top ups, I realised that performance started getting sluggish, even with the tablet.
The fuel consumption also went up.
I decided to do without the additive, the car felt dead.
So I switched to STP octane booster & carb cleaner to the petrol tank, and added engine oil additives too, but nothing helped.
I finally scrapped the car as the timing often went off (never happened before I used additives) and the car started consuming engine oil at an alarming rate (no leaks but have to top up once a week). Was using thick 10W50 mineral oil.
I spoke to several motorists and the general feedback that I get from thsoe who had tried additives is that it gives a "feel good" psychological effect, that's all, and does not perform as advertised and even caused engine problems in some cars.