You're dealer wants to make money. If you're an "average" driver, mostly highway miles, changing the oil every 5k miles is just fine. Save your money. By the way, don't by the marketing bull about using premium gas either. USA Today, engineering professors at UC Davis, a scientist at Lawrence Livermore National Laboratory, Wall Street Journal and others say premium gas a waste of money, unless you need genuine high performance, and Lexus' are not genuine high performance vehicles. Even then the difference is insignificant to today's engines. All "91 octane" means is that the fuel burns faster than 87 Octane, sometimes not desirable like when driving at high altitudes (1 mile or more). Todays cars cannot tell the difference between 87 or 91 octane. I've been using 87 octane from Costco or Rotten Robbie or USA Gasoline since I bought my IS350 brand new and there's nothing wrong with the way it runs. Don't believe me? Have family or friends do a blind test by having them put gas in your car multiple times (to eliminate guessing) and then you drive it trying to guess which fuel.
I can not vouch for the IS250 recommending premium and really needing it but it does play a factor. Myself and my crew run GST Eclipse and WRX Impreza and unleaded is abuse. It causes horrible idle and just ugly on the turbochargers performance. Higher octance withstands higher compression before detonation, so i would not be cheap cause if the motor is designed to run at higher compression and you use low octane don't be surprised if it causes engine knock later on. It's designed to run higher octane not to be expensive but to not have preignition or cause air/fuel struggle.
Maintenance Schedule For Is250/350
in 07 - 13 Lexus IS250 / IS350 / IS-F
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I can not vouch for the IS250 recommending premium and really needing it but it does play a factor. Myself and my crew run GST Eclipse and WRX Impreza and unleaded is abuse. It causes horrible idle and just ugly on the turbochargers performance. Higher octance withstands higher compression before detonation, so i would not be cheap cause if the motor is designed to run at higher compression and you use low octane don't be surprised if it causes engine knock later on. It's designed to run higher octane not to be expensive but to not have preignition or cause air/fuel struggle.