Don't some motorcycles use the engine oil to also cool something else? Transmission or clutch or something?
Both oils will be the same when cold, but the 40 will be thicker when warm. If it calls for 30, then the ring clearance and tolerances are built for 30. If you use 40, it may be to thick to get through and lubricate everything properly. The only way I personally would even think of usung 40 is if the engine has a lot of mileage on it. Like when the rings were worn down a little and even then I would have to see how much oil it was burning with 30.
How old is it? What oils were previously used in it? I've heard of engines that had used conventional oil all of its life and wgen switched to full synthetic after it had a ton of miles on it, the synthetic cleaned out sludge that was partially blocking a worn valve pan gasket, and the synthetic washed the sludge out and more or less opened up the oil flood gate.
I've seen some engine manufactures recommend not to use synthetic until a certain number of miles until the engine had time to break in. Supposdely synthetic is too slick and doesn't allow proper seating.
I've personally never been a fan of synthetic oil. I stick to conventional and synthetic blends. Synthetic blends are conventional based, with a little synthetic oil added. Usually around 75/25% to 80/20%. Unless you're towing heavy loads all the time, live in 100+ degree weather and/or pulling 5k+ RPMs a lot, I don't see the use of full synthetics. True they have longer oil change intervals, but I still prefer changing every 3k miles. I may get lazy sometimes and let it got upwards towards 4k miles, but not very often.
Have you been looking just in auto parts stores and Wal-Mart's? I would personally try to find a motor cycle shop of the brand of motorcycle it is, and see if they carry it.
yeah bikes have wet clutches so the oill has friction added lol it is crazy i thought oil should be slippery .im crazy about changing oil i do it at around 2k and sometimes ill change it in 20 miles becuase im afraid that not all the oil drained at last change
Check and see if your motorcycle needs JASO-MA oil. JASO oils are meant for 4 stroke motorcycles with wet clutches and do NOT include friction modifiers. We dirt bikers know all about motorcycle oil, but you didn't mention what motorcycle you are talking about. Street or dirt, my guess is your manual calls for JASO oil. Regular oils don't work as well with the clutch. If you ride a mellow street bike you probably won't notice, but off road or performance street, you'll notice.
Also, not a huge difference between 10W-30, 101W-40, 15W-40. I would run any of those in a bike that calls for 10W-30 if you can't get 10W-30. If you go a little high, your oil may run a bit hotter and you won't get maximum HP, but a 10 weight change for the "hot" viscosity should not hurt your engine.