My 2 cents on this subject: the optimum oil for your engine is the one advised by the manufacturer. Usually it is 20w in new cars.
Going for more expensive oils in 5w or 10w ranges does not necessarily translate into better engine protection, performance or fuel average. It that were so, the manufacturers wouldnt be so stupid not to advise these oils.
5w and 10w have lower viscoscity, they are thinner in flow. Their practical advantage is for countries where the ambient temperature remains below zero degrees Celsius esp in the night. These oils do not freeze at such temperatures, therefore the car starts easily.
Regarding synthetic/fully synthetic oils, again my logic is that if they were beneficial for your car the manufacturer must have mentioned so in the manual. At least as an option. I feel these oils provide far better engine protection, but not for conventional/commercial cars in domestic use. Engines like F1 which are high performance and low life (need overhaul after 500kms!) benefit most from such oils.
I can vouch that better oils give a better feel when driving 
But I also know it is mostly psychological Something like "A clean car runs faster".
Use standard oils from genuine vendors, replace at 4000-5000km.
Once I used Mobile1...only because the vendor told me that you can replace it after 15,000km. In retrospect, I dont think it was a good idea.
Currently I am using Shell HX7...vendor recommended 7000km...but I think it should be replaced at 5000km because it doesnt say otherwise on the box/packing.
Beware of marketing tactics. Use standard products.
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