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Toyota recommends premium fuel for the Sienna. While occasionally using lower-octane gasoline can save money, it lowers performance and could cause serious long-term engine damage that would not be covered under warranty. It is best to follow the manufacturers advice for the most part.
Premium Fuel
Toyota recommends premium fuel to boost horsepower and performance. The Toyota Siennas engine is optimized for gasoline with octane ratings of 91 or 92. This octane requirement is determined by the engines compression ratios and the vehicles performance needs.
Risks
If you use lower-octane gasoline, you run the risk of damaging your engine due to detonation (improper fuel ignition), which leads to piston and valve damage. This damage would likely not be covered by your Toyota Siennas warranty. The damage is structural to rings and pistons and requires an engine overhaul to repair. Having said that, youre more likely to suffer diminished performance rather than having to actually overhaul the engine.
Time Frame
Using lower-octane fuel on occasion is unlikely to cause any short-term major damage unless you hear engine knocking while driving. Using a mid-grade octane of 88 to 90 rather than regular (87), gives you the best chance of avoiding knocking. Any damage from lower-grade fuels would only develop over the long term.
Misconceptions
You may not actually save money by switching to lower-octane gasoline. This is because gas mileage can be reduced from lower performance, offsetting the lower price of fuel. You should experiment by tracking your miles per gallon for several tanks full of gas by using premium for some fill-ups and a mid-grade for others.
Time Frame
A consideration with all automotive care is time frame. The longer you intend to keep you car, the better care you should take. The type of fuel you purchase falls into this category of decision. "Cheating" with lower octane fuels is unlikely to harm a new Sienna in the short-term. Generally, follow common sense. If your engine knocks, it needs more octane. If it doesnt, mixing in some lower-octane tanks can be fine.
