It's 15W50 isn't it? i have read a lot...some technical and some forum threads. The concensus seems to be that M1 for cars should be just fine. M1 for V-twins is slightly different with very slightly different addiatives and specially made in a 20W50 to match harley reccomendations.
It all comes down to cost. the bike M1 is more expensive for no real good reason. If you use a lot, use the car oil. If you only ride enough to change once/twice a year, spend a few bucks and use the 20w50.
The difference between car oil and motorcycle oil is motorcycle oil has a sling additive to it and the car oil doesn't. Even though you are using syn. car oil don't be fooled that it is giving you more protection. Air cooled motorcycles run at higher temps. than car's do and oil manfactures formulate oil's for the difference in these temps. If you are on the road and need oil right now, then go ahead and use car oil, as a matter of fact Harley recommends that if you have to have oil right now and cannot get motorcycle oil then use diesel 15/40 oil. So listen up, use CAR oil for car's and please use oil's in you cycle made for motorcycles. Do some research on oil's and you will see why.