A somewhat related question I have had for a long time - is it beneficial to use premium on older cars? I have a 85 190E 2.3, whose manual states 87 as the required octane. The possible reasons that I can come up with, to use premium, are:
1. Older engines have carbon deposits, so knocking may be more, so better to use premium.
2. Premium fuels may have more cleaning ability.
3. Premium fuels may give marginally better fuel economy, so some of the extra cost evens out.
4. Premium fuels have greater fraction of ethanol, so they burn cleaner, so they will be better for those 15 year old cat convertors.
Now, are these reasons just fallacious, or, put together, they may justify spending an extra 15 cents a gallon on an old car (for me it comes to about $100 a year extra).