it will run like ass in the IS, friends of mine tried it. Power, smoothness, mpg were all horrid. There are certain cars like the ES (camry engine which uses regular), LX (tundra engine runs regular) that can get away with it.

For the extra couple $'s it costs for a tank of premium, why would you consider not running premium?

I tried both and I notice the power difference. I stick with 91 (or higher).

I always use premium in cars that require premium. The last time I used regular was during the Northeast Sandy hurricane. Man, you almost immediately feel the power loss, and to top it off, the acceleration is sluggish. I feel that an extra 20 - 30 cents per gallon is worth it.

I also use BP stations. They are everywhere in NYC. They also have a credit card that gives you 5% back for gas. Basically every time I go to the pump, it is reduced by 10 cents per gallon.

lmao what!!

No they put Premium on the IS sticker because the engine has direct injection and a high engine compression ratio. Modern engines are very capable of using a lower octane because they can adjust for knocking and pinging but you do lose power and efficiency as well as reduce overall engine life and risk more carbon build up. The RX and ES, ESh and CT are the only Lexus's that say you can use Regular 87 because they are tuned for that. Everything else including the RXh requires premium.

Its really tricky when it comes to what gas to use. There are some cars that have been DYNO proven that using 87 and 93 make little to no difference. When I had my honda, the V6 motors that run an almost similar compression ratio actually made more HP on 87 than it did on 93. Other motors like Mustang v8, VW turbo 5cyls, etc; there have been dyno proven up to 6hp difference when using 93 than using 87. I havent seen any dyno proven results for our lexus motors but i have used both on the 250 and there was barely any noticable difference. for MPG, maybe 2 or 3mpg less city but barely any when driving on the highway. I still got 28 mpg. I cant really comment about the 350

My suggestion is to do some research on your own and make your own educated decision about what gas to use in your car. YES you can use 87 but manufacturer recommends 91 or higher for optimal performance.

i don't care what the tech says, i would never put anything but 91 in anything with 11.8:1 compression!
any why? it won't save you any money. what you save at the pump you end up spending with more frequent fill ups and worse MPG. might as well buy the good stuff and enjoy the performance and better running engine without the risk of pre detonation.

In Australia there is a label on the filler cap that reads 95 or higher octane. Our regular pump gas here is 91 and there is usually 95 and 98 available.

I use BP or Shell 98 in my car. Mind you, it's 45 Celsius (113*F) here in the summer so a fuel more resistant to detonation isn't a bad thing.

I figure there is probably a good reason why they put it in the manual....My wife has an RX and it specifically states that 87 can be used. I don't think they would make it 91for the 3IS just because. I think it goes back to "just because you could doesn't mean you should".

Basically took the words right out of my mouth. Maybe im just old fashioned but if a car says "premium fuel" i just run that. that extra 3-4 bucks im saving on a tank really wont kill anyone.

But like hoovey said, these cars are designed and factory tuned for premium fuel. Can you run lower octane fuel, sure. but the result is your car wont perform as it should... (whatever salesman told you "its because its a premium brand" should be fired)

Wow, thanks, Bass! I totally forgot you can just look in the manual for ANY car and find the compression ratio.

Armed with that and Google, anyone can find out what grade gas is REALLY appropriate.

I wouldn't dare put 87 in mine. Sure, I could do that to save money. But if I was that concerned with not spending a lot on gas, then I would've gotten an economical car, not an IS. It makes no sense to me to limit the car's performance just so I can save on gas.

I just finished my 4th tank of gas. The first time was filled from the factory. Got around 22mpg. 2nd and 3rd time, I never noticed I was filling regular just due to the nature I've always pumped regular by nature, so I never payed attention. Got around 22-23mpg. This time around, I used premium again and got 22-23mpg.

Never noticed any sluggishness or knock or weird engine noises... *shrugs*

Page 510 in your owners manual tells you what you need to know. Everyone else is stating what they think and not know.

Because modern engines can adjust for knocking and pinging