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The check engine light would probably be on in this situation.

The check engine light would come on if the O2 sensor detected too much oxygen, indicating a misfire. It might not come on as long as engine management can compensate with timing adjustments to eliminate the ping.

...putting premium will bring back some power and fuel economy, but not to the original level with clean injectors, reset electronics and regular gas.

The customer is not comparing performance to clean injectors, because the deteriation of the vehicle's state of tune has evolved over several years. What the customer is comparing is performance with a tank of premium, vs the preceeding tank of regular.


Turbocharged Volvos, along with VWs, Chevy Cruses and such bring another variable, compression. Engine management can respond to detonation by opening the wastegate, which lowers pressure in the engine, and lowers power output.

It's easy to see how the typical driver, with a neglected car, can think that premium provides better performance, because it's a bandaid, but it doesn't solve the real problem.

I stay away from the semi-synthetic or full synthetic oil.

Automakers have gone to synthetic oils to advertise extended change intervals. Saying a car can go 10K miles between changes reads to the buyer as "cheap to run". My Ford, for instance, gives 7.5K and semi-synthetic as the change recommendation, for the US, but 5K and mineral oil for Canada. I remember in the 70s, automakers advertising changing the oil every 3.5K, but only changing the filter every 7K, and of course, not changing the filter, means that the quart of dirty oil in the filter immediatly contaminates the new oil. I remember TV ads specifically touting "how little maintenance" a car required.

What the cheap looking maintenance schedule does not tell the customer, is how exhorbitant the cost of changing synthetic oil is. At the Valvoline quick change place, mineral oil is $30, vs $50 for semi-synthetic. Changing the full synthetic in a VW reportedly costs close to $100.

imho, the extended change intervals are marketing hype, and they use synthetic oil to give the engine a chance of surviving that kind of neglect.

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