It's not so much that the manufacturers are inconsistent with the blends but rather the fact that retail outlets sometimes [DO] tamper with the stock. Refineries are highly automated and product composition rarely deviates from spec. Retail Outlets can blend - commonly using kerosene to increase profits (don't really know at what % concentration). That's why you sometimes feel that petrol taken at the same outlet varies in efficiency (km/litre). Maybe we can all go round and start mesuring the densities, s.g and vapor pressures of our fuels. Hell, shouldn't someone from consumer affairs or the trade ministry be doing this???
I feel that Petronas and BP give the best mileage. That's if it comes out of a retail outlet that does not tamper with stock.
V-Power (Optimax - Europe)has excessive detergents and sometimes washes out the engine too much. On older cars running on unleaded fuel, V-Power washes out the lead seals which in turn causes the engine to suffer in compresion.
Also, word in oil and gas circles - suspect Shell reduced "good stuff additives" level in regular unleaded to make V-Power feel very powerful. How else can you sell fuel at a higher price. Don't know how true but their straight unleaded sucks big time. Bad mileage and very noisy engine. May not be the same with other vehicles / ppl.
Projet if feel has a higher octane content, hence the power increase some are experiencing from controlled combustion and not pre-ignition under load. Contrary to popular believe, octane is harder to burn then other hydrocarbon constituents due to its higher activation energy (harder to start reaction) Which in turn leads to controlled combustion (once it starts!) which harnesses optimum chemical energy stored in fuel . Also there's this very strong ridsect/insect aerosol smell from the fuel. Not sure if this is what makes it better (could just be some sort of additive??).
Fuel is made out of a number of hydrocarbon cut points. N-heptane (designated 0) and isooctane (deignated 100) are added to modify the knock tendencies (pre-ignition) of the fuel. RON 97 would have the same knocking tendencies (under controlled conditions) as 97% isooctane and 3% n-heptane). So if the octane content is lower, theoretically the fuel should pre-ignite. This is where the RON number comes from (Research Octane Number). MON numbers (Motor Octane Number) is standard testing on engines with realistic loads througout the power cycle to stress the fuel more compared to RON tests. MON numbers would be 5-10 points lower than RON (but would give a better comparison of fuels). Having said that, if the car has excessive carbon build-up, the early morning starts with optimum octane levels would still be jerky and cause the car to idle poorly - so this I feel would not be a very accurate way of testing for octane levels in a fuel.
To wrap it up, keep mileage tabs on the fuels you ppl use. Listen for engine and accelartion characteristics of the fuel you're using. If you think a particularly outlet is dodgy - then stop filling up there. I'm sure you'll arrive at favourite brand/ blend. As for me, petronas and BP have my vote.