You must be very careful while selecting Engine Oil (Lube oil) for Honda Activa & Honda Dio if you are changing it. The Best performing Engine Oil (Lube oil) specification for Honda Activa & Honda Dio is SAE 10W30 JASO MB.
The above specification of oil is recommended by Honda website for Scooter (Scootee).
There is chances that when you go for oil replacement you will see SAE 10W30 JASO MA oil or SAE 20W40 type lube oil. Ideally you should not use 20W40 type of oil, 10W30 type oil is recommended.
See the difference of MA and MB in above two specification, you should not use MA type of oil, as it is best suited for motorcycle, which is having wet type of clutch, Honda Activa & Honda Dio is having dry type of clutch. There is difference in the performance of MA and MB type of oil, so you must stick with MB type oil (SAE 10W30 JASO MB).
Honda itself sell the oil of above specification with its brand name, you can find it in most of bike service center or Honda service center.
The cost of oil is around Rs 310 for 1 liter. only around 700 ml of oil is required for engine oil, rest oil can be used for replacement of gear oil (transmission oil) for Honda Activa & Honda Dio scootee.
Engine oil should be repalced in every 6 month or for every 5000 KM run whichever comes earlier, this will increase the life of engine and give smooth performace of engin. Many people don’t know that gear oil of scootee is also to be replaced idealy in every 2 year. The same engine oil can be used for transmission oil as well.
Other than Honda, there are some more brand which provide oil of SAE 10W30 JASO MB specification, such as Castrol Activ Scooter 10W-30.