FWIW, my Honda dealer here in Mpls told me the Honda brand 0W20 oil is SEMI-synthetic and NOT full synthetic. That seems logical to me because they charge $39 for a 0W20 oil change and to get full synthetic almost anywhere it cost you around $60 for an oil change. Full synthetic Mobil 1 0W20 cost $6.50 a quart at Walmart, so you are already over $33 just for the oil. Add labor and a good filter and your at the $60 mark.
Synthetic 0W20
Use Mobil 1 or equivalent.
Use only what Honda recommends which is SAE 0w20 Any major brand.
5w20 any premium brand. UNless you have a hybrid, the 0W20.
It varies from one brand to another but it averages around $7 per quart.
no synthetic, the clutch will slip.
Just use what Toyota recommends unless you live in a extremely cold place,then I'd go 0w20 because it flows better in the cold.
0w30
Yes, you can use synthetic, conventional, or a mix of the two.
Use 10w30 Synthetic.
Honda's 2005 Honda CRF250R takes 660 ml of 10W40 synthetic oil. There are several brands to choose from including Honda's own oil mixture.
If that is the correct weight recommended by Honda then yes you can use synthetic in the place of conventional lube.