S&S recommend a minimum of 91 octane in the workshop manual but don't state the units used so presumably it will be the American version so is equivalent to a minimum of 95 in Europe.
Given the M3W's tendency to pink, I run on super unleaded whenever available but don't actually worry too much about putting ordinary unleaded in if that is all I can get. There is a distinct possibility that the pinking is largely down to excessive heat in the cylinder heads due to the ultra restrictive standard exhausts rather than a high compression ratio so the problem may go away with a change of exhausts (or removal of the baffles) mine certainly pinks much more in warm weather than in the winter so heat seems to be an issue.
Last time that I was researching fuels was a long time ago and primarily for racing two stroke engines when the ACU had relatively recently mandated that super unleaded pump fuel was the only permitted fuel. It caused much wailing and gnashing of teeth amongst those who had their compression ratios and jetting set up for Avgas 100LL. Once people had sussed out the set up for super unleaded and learnt to not bother trying to read plug chops then power ended up much the same. The information I got at the time was that although Avgas is higher octane, it is designed to work in a relatively low compression, slow revving engine and at much higher altitudes than automotive fuels so isn't and never was the panacea for high performance cars and bikes that many think it is.
If you do want proper automotive high octane fuels then they are easily available, just not at filling stations and only at a price! Try Sunoco or WP-Motorsport for some eye watering prices and if you've ever been in a holding area with all the engines running this type of fuel then eye watering is exactly the description of the exhaust given off.
One of the big advantages of the race fuels is that they are as near as possible exactly the same batch to batch unlike any pump fuel which varies depending on time of year and what was available on the day at the refinery to make it from. It will also change depending on age as the tanks will not be kept completely sealed unlike the drums that race fuels are delivered in. Another big advantage of race fuels is that the organisers can tell you which one to run and a simple test can reveal if you are running something more exotic or are doctoring the control fuel. Not that I would have any experience of such things but at one time adding toluene or xylene to super unleaded allowed higher compression ratios so gave more power but wasn't detectable as they were only doing simple chemical tests on fuel samples that only showed that you hadn't added anything that wasn't already likely to be in the fuel. Later they started doing actual octane testing which meant more creative thinking was required and the cheating got more expensive. Later still the mandatory race fuels became compulsory in the higher level series which levelled the playing field somewhat.
