Originally posted by scanner
So mga sir : a fully synth oil with a lower API rating is still much better than a mineral oil with a higher API rating?
No doubt that synthetic oils resist heat and degredation longer than mineral oils but you also have to consider the additive package as indicated by its API rating. That additive package provides the performance benefit for the engine thus a mineral API CI engine oil will outperform a synthetic API CF engine oil.

Simply looking at it, the former has more additives which will take longer time to be depleted versus a synthetic engine oil which has less additives thus degrading its base oil as it gets affected by contamination and premature additive depletion during the same given period of engine operation.

Now in choosing the best engine oil for your vehicle, always check the manual for selected equipment manufacturer's specifications.