In response to the article found on DrDietThunder's posting, here is what I have to say.
Motorcycle Oil (MCO) vs. Passenger Car Motor Oil (PCMO) has been debated for years and as with most articles on lubricating oils this too has its flaws and merits. There are good technical reasons why motorcycle oil should be different from most PCMO's. Viscosity retention would be a reason but it is by no means either the most important or only reason. Also for viscosity retention, that only matters providing the minimum operating viscosity is maintained, especially at high operating temperatures. In this regard, any measurement of sheared viscosity needs to be made at 80 - 100C, usually 100C. Certainly not at ~40C as performed in the article. There are plenty of good oils available as far as viscosity retention is concerned and if a synthetic oil were better in this respect it would likely have more to do with the blenders ability to use 'better' Viscosity Index Improvers in an expensive synthetic base than would usually have to be used in inexpensive conventional oils.
Perhaps the strongest case for using a MC oil vs. a pass car oil is that current PCMOs are heavily friction modified to enhance fuel economy. The friction modifiers in these oils may result in a reduction in the coefficient of friction in the MC's wet clutch that can lead to clutch slipping or related less than ideal clutch operation and resulting shifting issues. There actually is a motorcycle wet clutch friction test for motor oils, JASO T 904-98. If all oils marketed as suitable for MCs had to pass that test it would be truly meaningful. Of course not all MCs use wet clutches so for some makes/models this test would not be significant.
All else being equal, MC's are typically more severe on engine oils that are automobile/light truck engines. Many MC engines do rev much higher, run hotter and shear oil quicker than auto engines, so a 'better' may be desirable, and no one knows better than the bike OEM what the minimum oil requirements are. But the OEM can't be expected to test every available oil to determine which are ok or otherwise. So they promote what they know is suitable and of course can make a few bucks on as a service part. Certainly a typical low viscosity, fuel efficient oil (5W-20, 5W-30 & 10W-30) is unlikely to suffice for most motorcycles. The SAE grades most likely to be specified/required are higher viscosity oils such as; 5W-40, 5W-50,10W-40, 10W-50, 10W-60, 15W-40, 15W-50 or 20W-50. These higher vis oils also are less likely to contain friction modifiers since the 40, 50 and 60 grades will not achieve fuel economy status. But as these heavier grades fall out of favour for auto use, oil blenders are less likely to have non-friction modified additives available with which to blend MC oils. Consequently more and more 'so-called' MC oils will be nothing more than re-branded PCMOs. Some have been so for years!
Some of the reasons MC oils are more costly than PCMOs has only to do with economies of scale. Smaller volumes, unique packaging and labels, less efficient distribution and higher per unit marketing costs all help move up the cost. And MC dealers cannot operate on the low margins that mass merchandisers and automotive retailers are willing to accept for commodities.
Regards,