3rz-fe (2.7) Oil grade info please

South Africa
Good day chaps, What engine oil grade do you use in a 3rz-fe? I am running 5w40 syn. We have harsh climates +/- 40deg C at times. I have noted impressive gains on a friends 3rz with 5w30 syn in terms of fuel consumption. Are there in any negatives to running a 5w30 syn? is it within spec? Toyota SA specs say a 10w30 is in spec but a 5-w30 is for temps only up to 14deg C. From what I gather most engines are using 5w30 now or does it depend from engine to engine? My only concern is when I go up to Namibia and the temp hits 45deg C/115F, I dont want to damage here and I dont want to compromise engine reliability. Thanks