Actually honda says to u use oil with grade as higher as possible so that means if u can find a api SL or SM oils that will be perfect but these oils are hardly available here and are very expensive.. around 275 per liter..
now honda never says to use only 20-40w they say u can use multi grade oils there is a chart given on honda manual which clearly says to use 20-40w or 20-50w oil for all weather conditions..
Another problem with oils for bike is that cd 70 have a wet plate clutch system means the clutch is dipped in oil so if ur using oil for example synthetic oil that will destroy ur clutch plate in days simply bcoz that oil have a friction resistors and they wont allow a wet clutch system to work properly.. but good thing is most 20w-50 and 40 oils dun have friction resistors... they are found in lower grade oils..
to me u should use atleast 20-50w or 20-40w oil in any bike and interval should be atleast 1500 in summers..
and leme tell u that now in new bikes honda says to use its own oil for cd 70 but change it after 1500 km..