Always a good question, and guaranteed to be 2 sides to it (and a middle...) EVERY SINGLE TIME it's asked. Nothing wrong with that.
Follow the oil recommendations in your owner's manual, and you'll be fine.
The oil manufacturer's recommendation for your GL 1800 is correct. There are often slickeners, friction modifiers, and specialized lubrication additives in some oils that may be detrimental to the operation of the clutch in some oils.
After 36 years in the oil and fuel industry and the lubrication additives business for the largest integrated oil company in the world, I can state categorically that no matter what one reads on the internet, the truth is that there ARE different additives in oils designed for cars/trucks/stationary engines that can be detrimental to your clutch and cause slipping - and therefore heat, wear, and damage - under certain conditions, that simply aren't an issue with an oil designed to be used in that application!
There's a reason they put - or leave out - different additives in oil designed for motorcycle use.
I've seen the tests and the results. Not many can say that. Doesn't stop them from saying it, though.
This is NOT to say that no one uses these oils in their motorcycles. Lots of them on this forum. And many have had good results, recommendations in the owners manuals notwithstanding.
My point is, the manufacturer has the info one needs to select the correct oil for one's motorcycle. If you follow their recommendations, if you ever need warranty work, you'll be on the good side of the bench. There definitely ARE ways to tell what kind of oil has been used in a failed engine.
You have a computer, you found the forum, you have motorcycles. You can do the research, you don't need to listen to what some nameless, faceless, 'expert' says on some forum. I'm talking about myself here. Make a considered decision!