Well, if the thread starter's MW3 does not suffer from pre-ignition/detonation and has a modern engine with knock sensor set up for 96, I cannot see any benefit in putting in anything higher - why try to fix a problem you do not have. But then there is the ethanol issue. As far as I know 96 octane has 5% ethanol mixed in, whereas 98 has zero. True, ethanol can act as a solvent and be detrimental to plastics, rubber, gaskets and so forth. But I do not think this applies to a modern engine. I suppose the engineers are very well aware of what is put into the tanks. On older cars or veterans it might be an issue though. And ethanol is hygroscopic, true. To this day I have never heard of a problem related to (these obviously small amounts of) water in the fuel, but thinking about it, I am considering filling the tank with 98 for winter storage but stick to the specified 96 for everyday use.

In older times the compression ratio almost dictated the the grade of fuel needed. In the 60/70/80s I used to run a TR engine in my race +4 at 12.5:1 and it demanded 5 Star (101 RON). However engine management systems have been transformed over the last decades to computor control of ignition and fuelling so the fuel demand can be very different

I think that all European vehicles from the last 20 years must be capable of running on 95 RON unleaded, not sure of the regulation but no doubt could find it given a few days of searching!

Most newer vehicles will have knock sensors (as pointed out above) and these were specified to ensure that "high-speed knock" (which is different to the "pinking" or "pinging" which occurs at lower engine speeds) did not not destroy the engine. This was a particular problem in Germany when cars were held at high speeds for long periods on the autobahn (especially prevalent in southern Germany on the A8)

However some performance engine manufactures have used the knock sensors to optimise engine performance based on the higher octane fuels (97/98/99RON) and thus extract more performance from the unit. So if you have such an engine in your vehicle, and are after the last drop of performance, then the higher octane fuels can make sense. For the rest of us, we are potentially wasting money by purchasing the extra octane.

Not an M3W but I find the 99octane (usually V-Power) does seem to improve things. Plus8, Rover V8 with Gems management. I was occasionally getting a random intermittent misfire fault. Checked all ignition and as a final test changed to the 99 from 95. It worked. I don’t feel any performance/ power improvement. Not doing a high mileage means extra cost is not an issue. I put 95 octane in if no 99 is available.
I think the benefits do vary with engine type. A mate has tried it on an outboard motor and spotted less vibration when used.
As an analogy I used to commute 60 miles each way, mainly motorway. Had a Focus turbo diesel. I experimented with Millers additive ( advertised as an octane booster). It worked. Using the recommended dosage from the bottle I got enough extra mpg to save about £24 per bottle on fuel cost (a few tankfulls per bottle). Bottle cost about £12 and extra mpg gave about £36 in fuel saving. A mate tried it on his Golf but no difference.
It could be worth giving the 99 octane a try over an extended period and take notes before and after. Compare mpg, how engine runs, similar mileage/route to get a decent comparison by eliminating variables.

As other's have said the higher octane fuels can be required by older high compression engines or offer a small performance advantage to modern ECU controlled engines with Knock Sensors. These allow the ignition to run slightly more advanced to the benefit of power and performance.

For the M3W I doubt (and have never felt) any performance benefit but tend to use V-Power or equivalent where available as these fuels have an enhanced additive package which helps keep the fuel system and injectors clean.

As an aside, Formula 1 rules specify a minimum of 87 octane and is typically around 91 octane so octane rating itself is no guarantee of performance.

The Kent engine in my 4/4 has a minimum octane rating of 97 RON. I use a lead replacement supplement and 14 years ago not all services stations had premium fuel so I used the octane boost version Castrol Valvemaster Plus to meet the minimum. Tom Airey, who tuned the engine for me and does many race and rally engines, told me that the racers generally used a double dose of booster giving an increase of about 4 RON, anything above that had no further benefit.

I have no trouble these days getting 97 Octane fuel and I noticed last year that Valvemaster still do a Plus version but it no longer provides an octane increase.

Some years ago someone, can`t remember who, did a comparison between 95 and 97 fuel in the uk.
If i remember right the 97 gave an extra 10% mileage. In theory its richer so you use less, is this correct?

On the other hand, in Malaysia they have genuine 100 ron courtesy of Petron Blaze. A test was carried out using motorbikes on a run from Ipoh to Penang and back. The result was very good with improved fuel economy and better combustion leading to a smother ride and better throttle response.

Is there a mention of the S&S manual recommending 100 ron?

I run my +8 (John Eales engine) on 97/98/99 I buy it mostly at Supermarkets where it is a lot cheaper than V power. Have bought 102 a few times in Germany and Austria in recent years.
Also my MB GLE43 specifies 98.

The S&S manual says to use, " a minimum octane rating of 91. Do not use fuel additives and avoid using methanol blended fuels. Ethanol blends of up to 20% may be used. Do not use E85 fuel in your S&S engine." It doesn't mention what sort of 91 (RON, MON or PON?) but I assume it is PON for the USA and that is about equivalent to 95RON for Europe.

The new Esso 99 Octane (RON) is 5% Ethanol, it has replaced the previous 97 that was available at my garage. I probably used the 97 out of habit having run classic motorcycles where I found it made a good choice. I believe the government has committed to continue the availability of E5 for the benefit of classic vehicle owners, otherwise we might all be on E10-15 by now.

Somehow people always translate higher octane equals more power.

That is not true.

Octane is simply a measurement that indicates how quickly the petrol will ignite. Higher octane ignites slower than lower octane. When vaporized fuel is put under pressure with ambient heat it will eventually ignite. That is how a diesel engine works. So higher compression engines need higher octane fuel or they will pre-ignite.

Now it is often the case that higher compression engines are also higher performing, or have more power. But that is because of the engine, not the octane of the fuel. If you put high octane fuel into a YUGO, you still only have a yugo.