I beg to differ. 5W-40 works just as well as any oil in high temperatures, its a multi weight oil, thus the 40 rating. What you are describing is a straight oil. If it were a straight 5 weight, then you'd be correct, it would be too thin at running temps. But having the 40 means that it works just as well as other 40 weight oil at high temps.

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1970 280SE Coupe W111
Chassis Number: 111 024 12
Engine Number: 130 980 12
Exterior Color: 180 - Silver Grey Metallic
Interior Color: 243 - Leather Light Red
Transmission # 004470