Yeah, even i was thinking on that to go for the 20w50 amosil. but somewhere i read that Higher Viscosity Index for Synthetics is not good always, means it would be thicker compared to a mineral oil 20w50 Film thickness. If that is the case, wouldnt my mileage drop further?I would definitely use synthetic. And, when you buy oil, you buy whatever grade you want. You are not forced to buy 5w30. Find a 40 or 50 grade synthetic oil, 5w40, 10w40, 10w50, 15w50, or 20w50... and keep driving. Shops/mechanics don't tell me what to buy and I don't care about their recommendations. I tell them what I want and that's it. You might not find every grade with every brand. So, shop other brands too.
I haven't seen automotive synthetic Castrol 50 grades in stock anywhere. I can usually order the 10w60, 10w40, and their 5w40's. Their website shows Castrol syntec 5w-50. So, you'll have to check to see what is available in your local store.
Amsoil does have Premium Protection 20w50 and Signature 5w50. These two would be easy choices.
Since you want to increase your oil change interval from 5k to 10k km, I would make sure that you check the oil level often and top it off frequently.
Again, i am new to these things and not sure what does a VI mean.