Yup, I'm goin' there. 

  So here's the current state of engine oils basic specs. I'm not going to go back into the history of specifications much, it's a long story and doesn't really matter any more.

  American Petroleum Institute.  https://www.api.org/products-and-services/standards

  Japanese Automotive Standards Organization.  https://360.lubrizol.com/Specifications/JASO

  General info on motorcycle specific oils, which gives a better explanation than I,  of the current availability of motorcycle capable oils. :  https://bikerrated.com/gear/maintenance/motorcycle-engine-oil-guide/


  Now, of course, the key to this situation depends mainly on what layout of motorcycle engine you have, whether it is a wet or dry clutch, and whether is is a shared engine/clutch/gearbox common lubricant. Some could take advantage of the highly capable and much less expensive turbo diesel oils, [Shell Rotella's line of T4, T5, and T6 diesel rated oils, all having some form of JASO motorcycle rating] ], IF your bike is capable of running the traditionally lower viscosity thickness oils for diesel vehicle engines, and IF you run separate gearbox and clutch to the engine. There are car oils out there of course, but most all are much too low viscosity, and most all have added 'friction modifiers' which are not good for clutch plates, your clutch will slip.

  Viscosity [thickness] is very important as well. It's thickness, based on JASO MA and JASO MA2 ratings, signifies it is made with longer chain molecules, which mean it will retain it's viscosity longer, under the normal sheering effects of motorcycle gearbox gears. In theory, non JASO MA/MA2 oils of similar thicknesses will last only 10-20% the mileage of a full JASO MA2 oil.

 

  There are oils out there specific to motorcycles, often found at motorcycle shops, which have no rating classification. It should be noted that oil companies pay either or both API and JASO to test their oils, thereby giving the customer the visuals to confirm they have been independently tested to meet criteria. I choose to buy oils from companies that have gone that last step of independent testing.
 
  There is the odd old motorcycle brand [Castrol GP non synthetic] which has the old API spec SG, which after spec SG, is kind of where car oils and bike oils started going down different paths. API spec SG has high zinc for top end lubrication. Most inventory of Castrol GP has now been replaced with Castrol Go!, which now has testing certification API spec SG, and JASO MA2, the highest JASO motorcycle rating.

Yup.  T4 in my bikes.

Rotella here too.

   I run different oils for different machines, based entirely on the machine's needs.

  Honda CT70 - 20W50 Castrol Go non synthetic.
  Norton 850  - 20w50- Castrol  Go  "       "\
  Laverda       - 20W50 - Castrol Go  "       "
  RC30             15W40  Amsoil  Ester based Full Syth
  BMW R100     20W50   Castrol Go
  Honda Xr650L 20W50  Amsoil  Ester based Full Synth
  BMW 1200     14W40  Shell Rotella   T4

I’ll use Rotella T4 15W40 in my CB750, and 10W40 (or whatever weight is available) in my Tracer900.  I’m not at all worried about running a bit heavier oil in the Yamaha.  Both bikes have a shared engine/transmission and wet clutch.  T4 is available in more weights tha the discontinued T3.

  The thickest viscosity found in the Rotella line is the T4 non synthetic,,,,15w40.

Castrol MC 10/40 in the 4 strokes and the cheapest 10/30 automotive for the RD/RZ trans/clutch.

The recommended oil for the 4 cylinder Kawasakis is 10W40.  I use Rotella 15W40, but I seldom ride in temperatures below about 15C and never above 40C. 

Almost all of my riding is highway, where engine cooling is usually best.