I recommend using only Jaso certified motor oils in motorcycles. Back when his bike was new, car oils were okay but today they don't have the additives a motorcyle engine needs to survive long term and they are too slick for the wet clutches.Regular Mobile 1 Synthetic 15w50 works great and it's a lot cheaper than their motorcycle version.
15W-50 is probably too heavy for an air-cooled In-Line 4 cylinder, especially up North. It might be fine during Summer in places where it gets hot hot hot.
I recommend using only Jaso certified motor oils in motorcycles. Back when his bike was new, car oils were okay but today they don't have the additives a motorcyle engine needs to survive long term and they are too slick for the wet clutches.
Auto oils today don't have additives like zinc that the old oils had and they don't protect the cams like the old oils did. If you start a newly rebuilt flat tappet cam engine with todays oils the cam will be wiped out pdq. Special breakin oils have to be used in those applications. Stuff like that is why I don't use anything but motorcycle oils in my bikes.
That research may be valid but it appears to be very old judging from the prices quoted for oil.
I agree but with a "but". For the engine, sure, I think 20w50 is fine even up north. The problem I've found is with the clutch- it tends to get "sticky" with heavier oils in the cooler mornings when in NC mtns. I notice it as warm as 50-55*F. When started in 1st gear with the clutch lever pulled in, the bike will lurch a little. Not an issue with 10w40 or 10w30.Going out on a limb here but guessing even 20w50 would be within original MFR's spec for the noted motorcycle - depending on ambient temps (especially for air cooled motors - not so much if water cooled). Gets very hot up north (Canada) in the summer (i.e. riding season) and running 20w50 would be perfectly acceptable. I regularly ran 20w50 in 80's inline 4 Hondas for years but have tended to go with the wider spec 15w50 since synthetics became more widely available.
And generally I think so too. I won't make a special oil change but if the timing happens to fall right, I'll use 20w50 in the middle of the summer. Otherwise 10w30 or 10w40 is fine for me. But (there's always a but) mine is a different engine. Being water cooled kind of negates the need for heavier oil as there aren't huge temperature swings like an air cooled bike.I've always just assumed the 20W-50 & some of the other heavier oils listed in the owner's manual were for Summer use or otherwise hot or extreme conditions, maybe racing, but the service manual doesn't mention the oil in the racing section.