Motorcycle oil 10W-40 is an 18,000-search US query because it is the default grade in a huge share of Japanese owner manuals. Viscosity is not the whole spec. A shared wet clutch needs JASO MA or MA2. Car oils labeled energy-conserving can make the clutch slip.
10W-40 covers most US street seasons. 20W-50 shows up on air-cooled V-twins and hot-climate Harleys. 10W-30 is in some newer manuals for fuel economy and cold starts. Follow the bike’s book, not a forum mix chart.
Change interval is miles or time, whichever comes first — often a year even on a garage queen. Filter at every oil change unless the manual says skip. Synthetic vs conventional is a budget and shift-feel argument; both still need the JASO clutch rating.
Walmart and AutoZone 10W-40 motorcycle bottles are fine if the label says MA2. Do not pour 5W-30 car synthetic into a wet-clutch motor because it was on sale.