The dealer told me I could put in Regular, I opted for the premium, BP 93
93 Octane fuel provides a 'better burn' - prevents detonation where lower octane fuel contributes to carbon deposits (piston surface / valves) that can cause early / uneven detonation in the cylinder impacting power. I also believe the higher octane provides a more stable fuel and is less likely to break down as fast as 87 octane.
The price difference for me between 87 and 93 is about $.25 - $.30 / gallon - which is not worthy of discussion.
I do have a problem with the $3.xx in front of the price per gallon. :-)
they are high compression motors almost 12.0:1, yes even the 250. They need premium. My friend has tried regular in his 08 IS250 and he said it ran horrible, bad mileage, bad power, sluggish
After my first tank of gas that the dealer put in it, I put in 87 not realizing that it required Premium. (I noticed the back of the fuel door as I was putting the cap back on)
I will say, it ran no different, made no less noticeable power (Lets face it, the 250 is a complete dog when it comes to accel.) and felt just as sluggish as a 250 normally feels.
I will be putting 93 in my car from here on out since I saw the sticker, as I don't need timing retarded and the car to be any slower than it already is .. but .. just saying, I had no immediate ill-effects after about 250 miles running 87.