I have used Susoco 260 for all my machines since we took a trip from Massachusetts to Foam Lakes Saskatchewan to visit our wild life biologist daughter. 4990 miles in 9 days in our 745.

I always write the mileage on all our gasoline purchase a slips. That way I can check gas mileage on a trip.

One stop in Quebec, I put the cheapest level of octane in by mistake (French gas pump). My gas mileage was 4 MPG lower for that tank!

Everyone thinks that the knock sensor will accommodate low octane. You know what the computer does when the engine knocks? It adds a lot of extra fuel to cool the cylinders. Changing the timing does not work because the engine if already firing before the plugs spark! Eventually it adjusts the timing.

I have never found a good explanation of exactly happens with a knock, but I have read about the extra fuel addition.

I would suspect a bad injector before I blamed low octane on an engine that old.

Make sure you read my other post before you spend money or energy.

I often keep my cars too long - I had to sell out '90 744 TI for $500 after putting that much on the car in new tires. My wife would not drive this beautiful car with no AC!