My manual (if i remember well) does say to use 91 as minimal octane.

It's really a waste of money to use the higher octane gas. It really won't do you any good, performance or mileage wise. Stick with the regular 87 stuff. I use Union 76 and have no problems with performance or gas mileage. Chevron is good gas, but the most expensive. I wouldn't use the cheap, discount gas 'cause it tends to be more dirty and watered down, so it doesn't burn as efficiently. I've noticed gas mileage differences between AMPM gas and Chevron.

Actually, because higher octane retards a fuel’s ignition, it’s use in engines for which it was not designed can actually lead to increased deposits due to incomplete combustion. If your car calls for regular, that is what you should use. In a N/A 850/S/V/C 70 motor, this probably wouldn’t happen because, if memory serves, these motors run a fairly high 10:1 compression ration. Anyways, you should use a fuel that has a proven deposit control package (e.g., Chevron/Texaco).

If you have a newer multi-coil ignition engine, like I have in my T5, it doesn’t really matter what grade of fuel you use. The computer-based ignition system used in these motors is sophisticated enough to control turbo boost and ignition timing to completely prevent pre-mature ignition (knock and ping). The only thing you may notice is slightly decreased performance (since the engine is being de-tuned by the computer). Around town, this will probably result in poorer fuel mileage. For highway driving, there will most likely be no mileage penalty. Personally, I use premium all the time unless I’m traveling on a long road trip (300 or more highway miles), in which case I use regular, and quite frankly I don’t notice a significant difference in performance.
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1999 S70 T5

Using a higher octane gas than needed is a waste of money. Lower octane gas ignites at a lower temperature than hi-test. In high compression engines low octane gas will ignite before the sparkplug fires causing a pinging sound. If your owners manual does not recomend high octane gas and your car does not have a pinging sound coming from the engine when under strain stick to regular. Twenty years ago when most cars had carborators the gas companies put additives only in their premium gasolines which were needed to keep the fuel injectors clean. Therefore, if you had a fuel injected car the recomendation was to use hi-test whether you needed the higher octane or not. Today all major brands put fuel injection additives in all their grades of fuel.

I tried a few tankfuls of 93 with my '94 NA 850 and felt no difference whatsoever so I went back to 87. I wonder what repairs could be avoided on these cars by using higher octane fuel?

if you have a high compression engine, then you have to use super

I used Mobil Super in my NA 855 since day 1 (new car). I am not sure about the argument of "fewer repairs down the road" but so far so good.

Ching-Ho Cheng
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1995 855 NA, Bilstein Shocks, K&N Filter, E-Code Lens