According to the official vehicle user manual recommendation, the BMW 5 Series should use 95 octane gasoline. Due to its relatively high compression ratio, it is recommended to use 95 octane unleaded gasoline for the BMW 5 Series. While 98 octane gasoline can also be used, it offers lower fuel economy. In addition to checking the appropriate gasoline grade in the vehicle user manual, you can also find this information on the fuel tank cap, which will be clearly marked. Typically, the gasoline grade can also be determined based on the engine's compression ratio. Vehicles with an engine compression ratio between 8.6-9.9 should use 92 octane gasoline, while those with a compression ratio between 10.0-11.5 should use 95 octane gasoline. However, with the application of some new technologies, the gasoline grade cannot be solely determined by the compression ratio. High compression ratio engines can also be tuned to use lower octane gasoline because, besides the compression ratio, other factors such as ignition timing, turbocharging technology, and Atkinson cycle technology also play a role. Generally, the higher the gasoline octane number, the higher the octane value and the better the anti-knock performance. 98 octane gasoline contains 98% isooctane and 2% n-heptane, while 95 octane gasoline contains 95% isooctane and 5% n-heptane. If the BMW 5 Series occasionally uses the wrong gasoline grade, simply switch back to the correct grade after consumption. However, prolonged use of the wrong gasoline grade can have the following effects: For vehicles recommended to use lower octane gasoline, using higher octane gasoline will not cause damage, but the increase in octane value will alter the fuel's ignition point, leading to delayed combustion in the engine. This means both the engine's power output and thermal efficiency will decrease, resulting in poorer performance as a practical feedback. For vehicles recommended to use higher octane gasoline, using lower octane gasoline can cause engine knocking. Due to the significantly lower octane value, the gasoline's ignition point decreases, causing it to ignite prematurely during the compression stroke. If combustion occurs before the spark plug ignites during the compression stroke, resistance will arise during the upward stroke. This resistance will make the engine operate very unstably. If the knocking is imperceptible, it only increases noise without obvious damage to the engine. However, noticeable knocking indicates severe engine conditions, where vibrations not only affect driving stability but also cause abnormal wear on the piston and cylinder, potentially leading to cylinder scoring in severe cases.
