first thing you need to do is understand what octane requirement is. Simply going by a manuf's recommendation isn't always in your best interest, not so much for octane requirement but there's a big information gap on the subject.
You mentioned the same engine ( i know nothing about volvos) but there's a 9 year difference- 1985 to 1994! Many things change on engines from year to year, even though they're the same engine. One of the main things to look for is the engine's compression ratio. Higher compression ratios, 10:1 or 10.5:1 and higher will usually require 91 or 93 octane fuel. But, as an fyi, compression ratio doesn't always mean higher cylinder pressures. Valve timing also plays a big part, and this will change from year to year because it plays a big part with emissions. What this means is engine running 10.5:1 compression may be able to run on 87 octane whereas another engine running 9:1 compression can requre 93 octane. compression ratio is just a quick and dirty thing to go by. Other influences causing the requirement of higher octane are turbos and superchargers, because even though the motor's CR may be low at 8:1 or 9:1 the forced induction adds boost (pressure) which raises cylinder pressures and also adds heat. Also check and see if the engine's you're comparing have turbos, if one does and one does not then that is a big influence on fuel octane requirement. And even if both motors have turbos, one might run higher boost than the other which would most likely raise octane requirements. Another thing to consider is environmental temperatures and engine operating temperatures. Newer engines, because of emissions, run higher coolant temperatures, and that may cause a need for higher octane fuel. What an owner's manual won't tell you is simply running in hot weather can either cause a need for higher octane or prevent you from using lower octane.
Knowing whether you need to run higher octane, and knowing whether you can't run lower octane, is what you need to know. If it's cool outside, you drive easy, and don't run high boost then you can get away with using lower octane fuel and probably not notice any performance loss or mileage loss. But if you run full boost with the pedal to the floor every time the light turns green, and it's 120+ outside, then there's a good chance if you run lower octane fuel you will get significant amounts of pinging/detonation which can cause damage. Some pinging once in a while, especially under hard acceleration, is normal and won't hurt anything. But the degree to which it does ping and how often and hard will be up to you to decide if it's acceptable and whether you either need higher octane or ease off the pedal. I would say if you're on flat highway road going 60-70 and step on it a little to pass someone and it pings and you can hear it, would be an indication you *need* higher octane fuel.
The topic of octane vs engine performance has to do with the ability of the engine computer to retard or back off ignition timing. More igntion timing (advance) results in higher cylinder pressures = more power = more performance and even better gas mileage. When you run lower octane fuel and there's pinging, the knock sensor detects it, and the computer compensates by retarding ignition timing to reduce cylinder pressure in the hopes of preventing the pinging or detonation. This also results in less power = less performance, and possibly less economy but these numbers might be too small to even measure depending on how you drive. This is why you hear or read, 91 or 93 recommended or for optimum performance, but okay to use 87. This is just a rule of thumb, because the computer can only compensate so much... by retarding ignition timing only so much. This is on newer cars, with OBD-II computers. Not sure if OBD-I computers had that capability. For the car years in question, I doubt a 1985 car could, a 1994 is questionable. This could be another factor where the same engine can have different fuel requirements, as there was a huge leap in computer and sensor technology on cars from 1985 to 1994.
