Both grades 5W-30 and 0W-20 are mostly available as fully synthetic and relatively cost more than the regular 10W-40 grade.
Fully synthetic with the latest API ratings such as SN/SN+/SM are preferred as these contain additives that are claimed to extend engine life and prevent the formation of sludge.
Another advantage of going fully synthetic is that you do not have to change oil frequently. Fully synthetic can last up to 8000 km before it needs changing.
NOTE : Oil grade alone is will not do the magic if you are looking for fuel economy.