So I own a 95 sc400... the user's manual says to use 93 octane which they don't have anymore, so I've been using 91. It's getting expensive though, and I was wondering if I could get away with using 87 or 89 without doing any damage to anything. Thanks in advance for any advice.

Most mechanics will tell you unless you're speed racer there really isn't a need for the highest octane. I've owned a number of cars over the years that the mfg's recommended 91 or 93 (3.0 Audi Avant, L6 BMW, Mustang Cobra), but as long as their isn't any predetonation (pinging), the lower ratings are fine. I've been running 87 in my 95 SC400 without a problem. The only vehicle I ever had a problem with was with a 3.3 911 Turbo. I'm a pretty mild driver but noticed pinging upon heavy exceleration with that car using even 89oct.

I have a 1997 SC 400 and honesetly it all comes down to how you choose to treat your car. Using 91 gas is like eating healthy for yourself even though it is a little bit more expensive vs. 87 gas is like eating junk food all the time. You probably wont notice any difference (besides in acceleration a little) but over time could cause you some problems. Always go with what the recommended octane is. Could save you some repair cost in the future but just my opionion.

P.S. I hate paying for 91 too but hopefully it will even out with less repairs in the future. Tried to go cheap before and had to get my entire fuel injection syystem cleaned out and repaired in another car i used to own

The octane rating will have nothing to do with dirty fuel, gunky injectors, clogged fuel lines or polluted gas tanks. Very simply: It relates to the ignition temperature (Heptane, iso-octane, blah, blah, blah). The higher the octane rating the higher the ignition temp (i.e. 87 burns at a lower temp than 93). High compression ratio engines typically have a higher head temp, therefore the gas ignites before the chamber is sealed, blasting the valve train, causing pinging (valves being forced shut). It is also not an energy measurement, so you won't get better MPG with higher octane.

If you drive hard or with the car under load you may notice pinging, or "pre-detonation" with low octane fuel. The SC400 1UZ-FE engine is 10:1 compression ratio, which is pretty high. Your pedestrian sedans run typically around 8:1. If you are a leadfoot speedracer, you'll probably benefit from a higher octane rating in the SC400. I'm a very reasonable daily driver type, so 87octane does me just fine.

I do recommend buying brand name gas to avoid polluting the system. I have no favorite, but feel comfortable with Shell, Texaco, Chevron, BP, etc, but am uncomfortable buying from no-branded convenience stores and Bob and Malinda's shop and go...

So what Greenie is trying to say is get 87. I have 250K+ miles on 87 with no prob. After I do a tune-up; (sometimes everyother), I treat her to an octane boost; (why not, she treats me good!). We do feel a difference. So unless you just want the quick jolt; get the high gas. Or, you can get the the octane boost with the 87. It's cheeper.

FYI: So you go to the gas pump; Mostly have three grades; ie, 87 -90- 93. There are only 2 gasolines there; 87/93. If you want the mid-grade; it just mixes the 2.