I always use Amsoil and go with the correct weight according to the manual.
Wow. I haven't thought this hard about something for quite a while. let first of all preface my following statements with I am not a mechanic or engineer, just someone who thinks they know everything about everything.I would have to say that for the most part you are right. The operating temp is much higher than 40c/104f, and with that thought in mind the ambient temp shouldn't make much difference. BUT, with a higher outside temp the cooling system would have to work that much harder to keep the engine within normal operating temps, thus it is concievable that at higher ambient temps the engine won't cool as much. Case in point is that when driving the car in cooler weather and shutting off everything is off, but when driving the car in extreme heat and shutting off the cooling fan remains running for a bit to cool the engine. That would make me think that the engine is running at a higher temp in hotter weather.Isn't the ambient temperature only important for starting? Once an engine reaches operating temperature, the outside temp should matter much, right? An engine at zero degrees outside won't be 40 degrees cooler internally than one at 40 degrees outside, right?