I have been using a 1.3 flx since 2012 and fc is worst than my c200 if comparing engine size and performance.
C200 is 66 liter tank and flx is 40 liter tank.
C200 Full Tank is RM128 and FLX is RM78 (based on current price RON95)
C200 can get 450km and flx can get 280KM (both are city driving)
So i spent RM0.27 per km for flx and RM0.28 for C200 making the flx a whole lot more fuel hungry car than the c200.
This is what I am getting from the flx for the last 3 years up to last week when a proton service center mechanic ask me to change petrol.
All the while flx is using primax 95 and shell for the c200 (mostly on 97)
This is my conversation with him (from what I can recall after 2 weeks ago):

Mechanic: En. pakai minyak apa? Petronas ke?
Me: Yes petronas primax 95
Mechanic: En, jangan pakai minyak petronas kat flx all the time.
Me: Kenapa dengan petronas? Engine is more silent than other petrol.
Mechanic: Petronas ni lebih water contents dlm chemical mixture. Petrol fuel have expiry dates. Petronas bila expired, will breakdown into bacteria and sink at the bottom on the big tank (35000 liters) at petrol station. Bila new petrol comes for refill, petrol station biasanya tambah masuk je dalam tangki. jarang la petrol station ni clean the tank. jadi old bacteria will infect the new petrol.
Me (already pening with his explanation): Apa impact kat flx?
Mechanic: Oh, flx ni ada cvt and cvt sensors tu kat dalam engine (pointing to me where the sensors are, somewhere where the spark plugs goes in). Lama-lama pakai petronas or minyak apa2, sensors tu kotor cause of the high water contents. Bila kotor, cvt cannot read realtime petrol needed by engines. So cvt akan request more petrol than needed.
Me: (OOO thats news to me and sound logical too) Boleh tengok CVT reading?
Mechanic: See, CVT should be moving between 8-700mW pressure, kete en ni 412mW average. quite high.
Me: habis camne, x boleh reset ke? (haha belajar kat KL Proton Center - reset CVT if got FC issue)
Mechanic: Boleh tapi tak sampai seminggu sama jugak nanti.

(Ever wonder why your car after service has better fc but back to bad fc after a couple of week)

Me: So how to improve fc?
Mechanic: Senang je, drive kete en ni sampai lampu minyak warning keluar, pastu ISI CALTEX TECHRON?
Me: 95 ke 97?
Mechanic: 95 je
Me: Kenapa caltex?
Mechanic: TECHRON additive ni power en. Boleh clean apa-apa residue dlm engine. Pakai je full tank 2-3 times then sure ada improvement.
Me: Ok, saya cuba and tengok la. Kalu xde caltex, whats the next best petrol?
Mechanic: Petron additives is good too. En try caltex tapi kalau dah xde, pakai petron. Minyak lain sama je macam petronas.
Me: Ok, terima kasih bye and thank you

So he reset the CVT, and off I go to drive back to KL and did not refill (already have half tank) and when left one bar, start looking for Caltex on NKVE, lol, no Caltex anywhere. So i add another RM20 Primax la, reach home and asks my son to NOT refill until empty/warning. Apparently Caltex in KL is also very scarce but manage to fill a tank full of caltex in Setiawangsa. Then another tank after a week and off driving the car to Tapah with full tank of caltex.

Surprise surprise, I drove to Tapah and back to KL with full tank and still has 2 bars left. For the 1st time my FLX hit 420KM before refuel. Extra 120km after 3 years of driving. wow. before this, long distance, short, city always get 280-300 max.

So here I am sharing the story and after googling, found out that Caltex Techron is a very powerful and proven additives and exclusive to Caltex only. Please google for more info.

The mechanic didn't know that since 2014 Caltex has Techron Plus Concentrated that you can buy and put in your empty tank and mix with other petrol and clean your engine with a bottle of techron and full tank of petrol in your car. Now I am changing my C200 to caltex as well.

Wow, long story short, test your car with caltex techron plus for faster result cause if use just caltex 95, u need at least 2-3 full tanks to clean your dirty valves or sensors in the engine and on top of not easy to find caltex in KL. Maybe if u work in KLCC, there;s one near Tabung Haji or Setiawangsa. From google, it says u need to fill in techron plus every 5000km.
Good luck