Originally Posted by danielTRLK

Long answer is it won't run as smooth. Lexus wants that cushion feel....it's why there's so much rubber in their parts, etc. It's all in the name of comfort. Ever been inside the dealership where they have those posters showing how much more design they put into all their parts for comfort?

Look at your valves and pistons at 100,000 miles in a car run on regular grade fuel. They're gunked up and have deposits everywhere. Look at valves and pistons run on premium fuel from the big players, they don't look half as bad. Not amazing but not nearly as bad.

Here's where everyone is getting caught up. People will say, I put 93 in my tank or 87, etc, then say it ran great and I couldn't notice the difference. While those opinions are certainly fine, we can't quantify what comfortable is for you and what comfortable is for me nor can you quantify the long term ramifications in one fill up. When Lexus puts an engine on a bench cell for testing and the engineers come back and say, wow look at this reduction in vibration levels we just recorded using better fuel, these are all things that can be measured. These guys are spending their entire lives redesigning every component trying to ease out another decimal in NVH level reduction. If I remember properly, MB's V-12 can have a glass of wine at idle and it doesn't fall off the engine. The balance of the engine plays a huge role in comfort. An engine that is more efficient will be more balanced, and a whole lot more. To put in perspective, I once had an 80 minute conversation with engineers about how they wanted to change a race supplier in a bearing could reduce 0.09 decibels in the engine's noise output, not something any of us could ever notice inside the car. Tally up enough of these gains however and suddenly bam, you've brought down 10db, etc.

While Toyota and Lexus do not have the fuel dilution issue that some of the german brands suffer from, have you ever wondered why they're recommending throttle body cleanings and fuel injector treatments? Those fuel injector treatments and throttle body cleanings are also tacking the valve deposits.

When you start to get deposits in an engine, you can see huge measurement differences on dedicated equipment that shows vibration and noise emissions. At the end of the day, Lexus touts itself as more reliable, comfortable and luxurious than Toyota and all others. Using premium fuel helps achieve this goal.

2. Probably because the marketing department fired back that the competition isn't requiring premium. At the end of the day, you have to sell product and marketing departments usually get the final say. The RX is not in the same realm as the GX or LX. The marketing departments for the GX and LX are assuming people who own these cars won't have trouble spending an extra $10-20 per fill up since they're already getting 17mpg in a $70,000-$100,000 LUV.

I suggest to those that are worried or looking for a definitive answer here is what I would suggest......

Purchase an E85 kit or run E70 without the kit if you have E85 availability near by. You could alternatively use E30 if your station sells it. I'm still trying to boil down at what point the ECU says too much trim and throws the CEL but that plan has been put on hold for now.

1. Your engine will make more power and advance timing more than it could even with Premium. (E45 +)
2. You'll be supporting American made fuel vs supporting wars overseas and military budgets.
3. You will clean out all the deposits and gunk left behind by gasoline/gasohol in your engine on the pistons, valves and guides, etc.

If you don't want to purchase premium fuel, add a half bottle of VP Racing Fuel's system cleaner every 5,000 miles on regular grade fuel at a minimum. With good oil analysis you can determine how much you should really be using, but a half bottle is better than nothing.

If you don't mind paying for premium fuel, run the VP Racing Fuel's system cleaner every 5,000 miles anyways to really keep the engine tip top.