5w-40 fully synthetic is the best, but our everyday running cars don't need the best - they can do just fine with good. So what is good ? This is what I found out after spending some time on the internet a while ago.
Oil generally flows differently at different temperatures, but motor oils are designed so they keep a certain range of flow rate between normal engine running temperatures from cold start to heavy work load. Synthetic oils tend to have wider ranges e.g. fully synthetic 5W-40
5w is SAE 5 rating of the oil at 0? F (-18? C) - meaning at 0? F (-18? C) the oil will flow like SAE 5 rated oil
40 is SAE 40 rating of the oil at 212? F (100? C) - meaning at 212? F (100? C) the oil will flow like SAE 40 grade oil
Temperatures mentioned above are engine temperatures and not the atmosphere. So initial grade can be 0W to 10W. After few mins the engine reaches its normal running temperature that averages around 212? F (100? C), so the second number has to be 30 or greater to make sure engine stays lubricated at this temp. Anything less than 30 is too thin.
Avoid mineral oils for engines built 2010 onwards. Use synthetic oil, it does not have to be fully synthetic; semi synthetic works just fine and is best value for money. You use synthetic oil because it has better temperature range and not because it has some magic powers. Depending on the additives some synthetic oils last longer compared to mineral oils.
Look for SN grade but SM grade will do to.
To sum it up, the oil has to be synthetic, has be at least SM grade and the second number in (-W --) has to be at least 30.