Regarding the letter on motorcycle engine oils, you are correct about most motorcycles using a wet clutch and the need for extra additives in the oil, but there are a couple of other issues of importance.
First off, cars use a separate oil supply for engine and gearbox, which allows the oil manufacturer to customize oils for each application. Motorcycle engines on the other hand usually use a common supply for both, which means that motorcycle oils need the additives in them for both engine and gearbox applications. The problem is that being in an engine destroys the additives needed for the gearbox, and vice versa, which is one of the reasons that motorcycle oils need changing so often.
The other issue is that motorcycles tend to be in a much higher state of tune than a similar sized car engine, and hence work their lubricants a lot harder. This shortens their life too, and means that you need higher performance lubricants in the first place.
When picking an oil you need to ask yourself two questions :
Is the oil supply for engine and gearbox separate?
Is there a dry clutch?
If you can answer yes to both questions then good car oils will do the job. If the answer to either is no, then you really need dedicated motorcycle oils.
The Harley brand oil mentioned in the original letter should have a section on the label telling you what particular standards it meets (as will any oil). Look for the text talking about American Petroleum Institute (API) standards or service classes.
The service class is a two letter code, the first being S for spark ignition (petrol / gasoline) or C for compression ignition (diesel), the second letter identifies the exact standard, with B being more modern than A. Given that engine performance has increased with time, a B standard oil could be said to be higher performance than an A standard oil (very, very roughly). Standards develop with time, and many are obsolete, but so long as the oil you use meets or exceeds the standard on the bottle / in the owners manual then you are should be OK (i.e if you need an oil to service class SF, then oil to class SG will do, but not the other way round).
Bear in mind the points above though, if you have a wet clutch / shared oil supply you must use a proper motorcycle oil as a car oil will not have the correct additives.
I’d also be as wary of using really cheap oils, even though they meet the required standard. They will be cheap because they use cheap ingredients, which won’t last as long in the engine. Oils degrade as you use the engine, and a good oil will degrade less then a cheap one will.
As to traditional versus synthetic, well synthetic oils work better, lasts longer in the engine and can have a wider temperature grade – 5W50 against 20W50 (for example). Given that most engine wear happens when it starts (before the oil gets moving) an oil that is 5 weight when cold will be lubricating the engine a lot earlier than one that is 20 weight, so the synthetic oil will help your engine to last longer too. The down side of course is that synthetic oils are much more expensive.
The higher performance the engine, the more important this will be. A Harley (which is low performance by motorcycle standards) would see less benefit than the latest Japanese or Italian racer. I hope that the preceding is informative, and thanks for all the effort that goes into the site. – Brian E.