I live in Phoenix and when I get up in the am it is 80+ degrees! So I just changed my oil yesterday to 50 wt. racing oil- the rest of the year I use 20w50. Daytime temps are always above 100 degrees for 5 months. Car has 280k on it- so far so good( like the man said when he jumped out of the twenty story building). The oil guage stays pinned at idle with the 20w50. BTW, I also crank the motor to get oil pressure by not activating a switch that turns on the fuel. Once the guage says I have full oil pressure(I know the thing (tensioner)that keeps the timing chain from slapping has pressure) I hit the switch and light up the motor. Three summers I have driven this old Merc like a taxi! just like in Europe.