There is a chart that shows the effective ambient temperature operating range of oils and 30w does not cut it if you are in a hot area , I occasionally have to mow when it is round 40c/100f and 30W is about as good as water
Make your decision based on the chart
Oz
What does 40C have to do with engine oil temperature? IIRC typical crankcase oil and engine temps are more like 90c+ . Is there a disconnect in reasoning here?
As mentioned, IF the engine is running "HOT" it's not the oils fault. nor will it be the cure.