I really like the newer 5W50 synthetics for covering a wider range of temperatures. It seems to have the best of both worlds.
The W50 part is best for air cooled engines that get hot when working hard even in cooler climates, but especially in hot climates. It doesn't break down, and maintains its film strength and lubricating ability at high temperatures.
Also, the 5W part is better for cold engines during cranking and warm up. It is thinner when cold so that the starter and battery don't have to work as hard to crank the engine, and it has a better film strength and lubrication ability while the engine is cold and during the warm up process, when a lot of the wear and damage to engines occur.
My experience with oils has been an evolution over time, much like the oil industry itself experienced, along with the evolution of the design and manufacturing process for engines and other rotating machinery. During my youth, everything used straight 30 weight oil in everything from chainsaws to diesel tractors. The only option was detergent or non-detergent. Manufacturing processes, strengths of materials, machining tolerances, and almost everything else were inferior to today's standards. Engines were shot, and needed rebuilding or replacing long before 100K miles. As manufacturing, materials, tolerances, etc. improved, clearances were tightened up, and demands on oils increased.
Multi-viscosity oils were developed to protect engines and work in hydraulic lifters over a wider temperature operating range to protect engines at both start up and high load, high temperature operation. This included operating in varying clearances as parts heated up and expanded, or cooled down and contracted with temperature changes. The evolution of designs, materials, manufacturing, and lubrication have resulted in engines that are expected to routinely exceed 100k miles. Many are now warrantied to meet this higher expectation. Engines now outlast the rest of the vehicle. Body, interior, and new features now dictate when owners trade vehicles rather than engine wear or failure.
Trucks are a prime example of the evolution. My father never got 100K out of a truck in his lifetime. My early trucks were worn out before 100k. Today, my daily driver has almost 150K miles. My brother-in-law's has around 250K miles. My brother recently donated his older truck to his son-in-law with over 350K miles and it is still going strong heading for 400K. He also donated a newer truck that he picked up used to his son with over 250K miles on it. We all want new trucks, but not because of any engine concerns. Interiors are shot, bodies are in various stages of deterioration or abuse, features are obsolete, or no longer work. However, they are paid for, mechanically as sound as ever, and the price of a new one is extremely unattractive.
Synthetics were a big part of the evolution and improvement in lubrication quality. Much research was done on improving oil film strength and film thickness. Additives were developed to improve everything about the lubricating and cleaning aspects of lubricants. One of the main drawbacks of dinosaur oils was the breakdown of its lubricating ability over time and temperature. Oils were changed at a maximum of 3K miles and more often under harsh conditions. Some people even changed their oil twice for every time they changed their filter. As oils improved, oil change intervals increased. Today, I believe every manufacturer of engines recommends synthetic multi-viscosity oils as the preferred lubricant. Some manufacturers require them in order to maintain your warranty. Many newer vehicles uses sensors to determine when you should change your oil. Oil change recommendations now exceeded 10K miles in many instances. Some people are now changing their filter twice before they change their oil. Manufacturers are trying to develop filters with higher capacity for holding contaminates in order to take advantage of the 15K and longer synthetic oil life.
My experience with a large manufacturer of heavy duty rotating machinery over a career that evolved from design engineer to test engineer to application engineer, to sales engineer to field services engineer to technical services and warranty manager, reflected this same evolution of manufacturing, materials, and lubrication. Working with lubricant manufacturers to supply and evaluate lubricants and their ability to protect bearings and other parts led to the development of testing programs and procedures for analysis. After years of using heavy aircraft grade petroleum lubricants, it became apparent not only from test results, but to the most casual observer that synthetics made a huge difference in the life of parts. While abuse, contamination, and loss of lubricant have always been a big part of industrial equipment failure, there were many that just wore out due to the lubricants not providing a sufficient oil film to prevent metal to metal wear. Today with the improvement in lubricants and changes to synthetic lubricants, almost all failures are due to contamination followed by abuse and loss of lubricant. No one ever runs one long enough to wear it out any more, since something else always gets it first.
With the Rebel, there is no oil filter, so the oil must be changed more often due to contamination from combustion byproducts. Any oil will last longer than my recommended oil change interval of 1-2000 miles. Even petroleum based oils will not break down from temperature during this short time interval. Correct viscosity to protect the engine over its entire operating temperature range is much more important to engine longevity, as long as these frequent oil changes are observed. With dinosaur oils I feel that ambient temperature affects the correct selection more than with synthetics. In cold climates, I think the new 10W40 blend is the best option for dinosaur oils. In hot climates, the 20W50 blend is probably a better option for dinosaur oils. With synthetics, I think that the 5W50 blend will cover both. Only in rare, extreme cold operating environments that are not friendly to humans would I think that the 0W40 blend is really needed. It will only be a matter of time before 0W50 is a more common and available option, and when it is, that will be my choice.
Dinosaur oils are cheaper, and if you are stretched financially, or just don't believe you should use anything better than what is minimally required, then that is what I would recommend for you. If you can afford it, and you want the best for your equipment, then synthetics are for you. For me the cost difference is not enough to consider an inferior option.
Regarding brands, during my years of working with oil suppliers to test and develop better lubricants for my company's applications, Royal Purple stood head and shoulders above the rest. Mobil 1 was a clear second. The rest were so far behind and so inconsistent that I could not recommend any of them. During that time, several of us engineers were involved in racing cars, boats, and motorcycles. We took the opportunity to test these lubricants also, and to pick the brains of the experts. The results were the same, and ended in all of us switching to Royal Purple in all our race vehicles, personal vehicles, and equipment. Not only did we switch our OEM supplied lubricants in our new equipment, our replacement parts lubricants, and our literature for OEM recommended lubricants to Royal Purple, we also switched to Royal Purple in our truck fleet and all other company vehicles and equipment. My data and industrial experience is somewhat dated, and things may easily have changed in the lubricant industry since then. Based on literature today, the rankings are still valid. Based on usage and volume of user feedback, Mobil 1 may be on top now, but that may just be due to being a larger company with a larger advertising budget, much the same as it was when I did my tests.
I do feel compelled to add that these are my OPINIONS, and YMMV.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
ALL IN! GO TIGERS!