As John says it's only a waste of money if your car is tuned to use 87 and you run premium. A stock 240 is a good example, it's tuned for regular, low compression and a mild spark advance, tossing premium in there is money down the drain. Kick up the compression or advance the spark and there is some benefit. Carbon buildup can necessitate premium to quiet pre-ignition also but that's an exception rather than the rule. Don't forget the placebo effect when reading threads about "wow my bone stock 240 really likes premium" or "my electric fan conversion gave me 2mpg"

It sounds like your engine was rebuilt before you bought it, without knowing what exactly was done all you can do is go with the seat of your pants and ears, if it likes premium go with it. Who knows what the rebuilder did, my Duett has a B18 bored to B20 (receipts for pistons to "prove" it) and still has the stock B18 head. Do I run premium, you bet, it won't let me do anything else, at least 'till I get time to put a late B20 head on it.

Sounds like you have all the bases covered, run the car on what it likes and enjoy it, it's a small price to pay for a good running car.
--
Dave Shannon
Durango, CO
'63 P-210
'67 1800s
'88-240
'06 F250 Diesel
my pages