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i've followed Randakk's advice whenever i can/need to, but this one caught my eye...
Use regular unleaded gasoline only. Premium fuel is not necessary or required. Don’t use octane boosters or lead replacement additives either. The GL1000 has hardened valve seats that are quite happy without lead.
i was always of the opinion that 91 as the one to go for? i appreciate this is as contentious an issue as 'which oil/tyres etc' but just curious

In a nutshell, higher octane fuel has a higher ignition temperature and burns 'slower' to counteract the issues associated with old school high compression engines. The wing engines don't need it, and it can actually cause excessive valve and combustion chamber deposits. Check out Mike Nixon's Spot in the Guru's forum for a couple of more in-depth threads concerning octane, fuel additives, etc...
FWIW, I run fresh, good quality ethanol (10% or less) in my bikes all the time with no issues. I also try to run through at least a tank full of fuel a week, and use Stabil when I think I won't be riding for a while... my 2 cents.

Track T is pointing you to read articles like this one from Mike Nixon.

https://www.motorcycleproject.com/text/octane.html

High octane is to prevent pre-ignition "ping" in high-compression engines like some BMWs and HDs. The GL1000s are not high-compression, and you should use regular gasoline ~87 octane. The high octane fuels will only result in more carbon build-up. Ethanol is avoided due to fuel system problems from to moisture attraction. Ethanol to 10% works Ok in fuel injected engines although it has a much shorter storage life. I personally buy 3 five-gallon cans of regular non-ethanol gas and treat them to manufacture's specs with Stabil. This is for all of my carureted engines - garden tractors, UTV, chain saw, string trimmer, lawn mower, leaf blower and 2 GL1000s. The GL1800 uses 10% ethanol with no problem as it gets ridden enough to prevent stale fuel problems. None of my engines are high-compression.

Its not contentious. There is a lot of misunderstanding on just what regular, mid-grade and premium actually are. Octane rating has nothing to do with power or available power from gasoline. Octane is a measure of fuel's anti-knock properties. (look it up). If you have a low compression engine than the grade of gasoline you need is 87. It is formulated to prevent pre-ignition or knock in these types of engines like the GL engines. If you have a high compression engine like found in Harley's and BMW's then you need premium grade fuel for its slower burn and higher resistance to engine knock. If you are using premium in a low compression engine you are actually likely to lose a little horsepower because the fuel will not burn as fast as the engine was designed for. Additionally, Ethanol is a great anti-knock additive because it slows the ignition rate of gasoline. Its the most environmentally friendly anti-knock additive as it does not pollute, or at least pollutes very little. Anti-knock compounds added to unleaded gasoline that does not include Ethanol are far more hazardous to the environment. For this reason alone you won't ever see gasoline/Ethanol blends disappear from the market until every vehicle on the road is electric.

I haven't done a deep dive into the technicalities of it all, but this is my understanding:

- Premium is only necessary for high compression engines (I believe anything higher than 9:1) and the manufacturer will make it clear. You're wasting your money if it's not specifically required by the manufacturer (or if you change head gasket, squish, etc.).
- Midrange should only be used if you have problems with knock on 87 octane. Again, you're wasting your money otherwise.
- Unless you have a specific reason for premium or midrange, use 87. On my '80 GL1100, the spec sticker says 91 ROM, which is a European (I think) measure. It equates to 87 (R+M)/2 used in the states, or maybe slightly lower.

None of this has anything to do with ethanol, BTW. Unless you're riding frequently, ethanol will turn the inside of your carb into sludge quickly. I've started going to the trouble of keeping a 5-gallon VP jug of 87 octane ethanol-free in the garage for my bikes. You can also get Sta-Bil and the like that are supposed to help counteract the effects of ethanol, but I've never used it.

The other negative that I see with ethanol is that it reduces mpg. So the ethanol burns cleaner as mentioned above, but if you lose 10% mpg, you’re going to burn 10% more fuel, negating the perceived environmental savings. When I checked tank by tank comparisons, I would see about a 7% drop in mileage in my Buick, 3800.

If we could bump the compression up, it would burn the ethanol more efficiently.

From what they’re saying today about oil reserves, I would guess we’re a long ways from pure ethanol.

right then. regular gas from here on in. thanks to all who replied !

I don't remember the last time I saw regular gasoline without ethanol for sale around here. Since I retired it often takes me more than a month to use up a tank of fuel (longer if I fill the tank on the sidecar too) and it hasn't "turned the insides of my carbs into sludge" yet. In fact, on the rare occasions when I have my carbs open they look a lot cleaner than the ones I see pictures of on the forums so it would seem to be quite the opposite.

Some claim that the ethanol has a negative effect on the rubber but I say that if someone is designing & selling a motor for use in a place where E10 fuel is common (= most of the world these days) they should take that into account.
The same goes for people selling carb kits for engines that were made before E10 was common; I can't say about any other suppliers but I know that Randakk has made every effort to do that and to stand by any of his products that may have had ethanol related problems.