What grade of engine oil would you recommend? Over here we have got very hot summers and very mild winters. Engine has got 80K miles on it + never been overhauled or opened. Currently it has got Mobil1 5W-50 (as reccommended by Ford????) supersyn fully synthetic. Engine runs smoothly+ no smoke or noises whatsoever. I could no longer find Mobil 5/50 locally so was considering using Castrol edge sport 10W-60 fully synthetic as well. As far as my knowledge is both are API classified as SM.

Thanks