I worked for an international oil company for 25 years. The last 9 were in their international lubricants technical centre. I find most of what has been written here pretty close to the mark, but there's still the odd misconception which creeps in.
One thing I have found is that there isn't a piece of equipment which doesn't benefit from a reduced oil change interval. If the manufacturer states oil change at 8000 km, I change at 5000 km. I pretty much change at 5000 km anyway, unless the manufacturer recommends a shorter oil change interval. Extending your oil drain interval is just false economy.
Use the highest specification motor oil. In motorbikes thats something like an API SM or SN plus a JASO MA qualification - the JASO MA addresses the specific friction modification requirement for a wet clutch, but the additive package is skewed towards a higher anti-wear additive package. As for the viscosity grade, the high temperature grade is determined by the internal clearances of the engine, the low temperature starting requirements of the engine determines what W grade the oil should be. So if the manufacturer recommends an SAE 10W-40 then use an SAE 10W-40. If you operate in a high temperature zone the manufacturer will have printed a graph in the handbook showing the appropriate engine oil grade to use for your conditions (Honda does).
That OP who asked about Delo, thats just Chevron's (and Caltex's) cover brand for a range of "Diesel Engine Lubricating Oil". I think there's a range of Delos ranging from Delo 100 (a low performance engine oil for older naturally aspirated 4 stroke diesel engines) to Delo 400 Multigrade which is a pretty high performance diesel engine oil for the latest diesel engines. Delo won't have the friction additive that addresses clutch requirements. Delo 400 Multigrade is a good engine oil for petrol engines too, but I wouldn't use it in a motorbike engine. Yes it does have a higher level of detergency in it's additive package, and therefore will clean engine deposits out and appear darker - any diesel engine oil will do that, they're just doing their job.
Friction modified engine oils (JASO MA) lose their friction modification as they do their job, that's one reason why I tend to change my motorbike oil at shorter intervals. Towards the end of their oil drain interval you will notice a drop off in gear change feel, this is the friction modification being depleted, change the oil and you will notice improved gear change feel.
Synthetics? Horses for courses. Depends on the additive package used. I believe the first Mobil 1 was a pretty shitty oil, I think the formulators thought the use of a synthetic base oil would reduce engine deposits so backed out some of the detergency/dispersancy package. They learnt quickly, and now Mobil's synthetic range of lubricants is top class, Mobil have made their synthetics range their flagship brand. In the USA manufacturers tend to include highly hydrotreated mineral base oils (Group III basestocks) as "synthetics" (Google Mobil vs Castrol and the Better Business Bureau), in Europe they tend to accept that only Group IV and V base oils (PAO, Esters, PIB, etc) can be described as "synthetic".
Motorbike engines need a specially formulated engine oil. They work at a higher specific thermal efficiency (they produce more power for their size), there's usually less oil in the sump so what is there is exposed to higher thermal and oxidative loads, and there are specific requirements of a motorbike engine/transmission which must be specifically addressed.
Use a specific motorcycle engine oil, use as high a performance engine oil as you can get, and change it at reduced drain intervals.
