Synthetic oils require little or no polymer, so viscosity change is very small. Cost of doing business is higher per quart of motorcycle oil. Large oil companies make so much more product that their profit margin per quart does not have to be so high. Some motorcycle oils may actually offer less protection than SJ automotive oils. That's a good one.
Chances are, there is no evidence.
Now, armed with all this information, you are ready to make your choice between automotive oil and motorcycle oil. Either will work fine. Your motorcycle probably cannot tell any difference. There are many riders, the author included, who use nothing but good quality automotive motor oils. There also are many who use nothing but motorcycle oils.