Yes...SAE 40 will be fine for the primary drive...The M20 primary chain case has a level plug and a drain plug...The level plug is in the front of the outer cover just below the footrest...
Remove the level plug and the large brass inspection plug. Put the bike (off the stand) on level ground and ensure it is standing up fairly straight. Then slowly pour oil in through the inspection plug hole until it just starts to drip from the level plug hole...Replace the level plug immediately...
I use a funnel with a bit of rubber tube fitted over the end and can thus allow the oil to run slowly down the back face of the inner cover until it fills the cases enough to reach the level plug hole...Ian
