will the old girls run happily on 91 gas?
Where do you live again?
Mostly going to depend on compresson. I'm not sure what the MFG said about the lower compression models (USA, esp later USA etc), but you could probably escape using mid-grade. Mine ran fine on 87 (pre-valve job, with gasket leak).
However, if you're in the US, the differece between filling with reg versus super is maybe $5-$10 on a whole tank.
The other reason to use super is that (in the US anyway), the top tier gas usually has more detergents in it. These cars are old, and the injection systems, valves, etc are sloppier than in brand new cars because they are not as precise. I do believe the cleaners make a bigger difference in older cars.
At XOM, our tankers of reg come in plain Jane tankers. However, when we get the load of super, it comes in a special tanker because it is more than nominally different. It cannot be sent to another brand station. Mid grade is often made by blending Super and Regular.
One more thing: the cost of using an octane booster with regular or whatever isn't usually worth it (in the US) versus using super because they are typically $4-ish a bottle. (ie the price of regular gas + booster is about equal to super gas on a total fill up). Of course, if the motive is to boost octane above the top gas available, or as a routine monthly cleaning, that may be different.
This may or may not hold true in other countries.
Gas is about $3.669 per gas / 3.78 litres for reg here in FL atm.
