at the same time if they viscosity is very low for an extended period of time its a bad thing
for example : 5w20 + 90 degree heeat + 5 hour drive might not be a good thing for obvious reasons
IDK, I'm using 5W-20 in my 04 Grand Marquis. I regularly drive from So. California to Arizona. It's about 6 or 7 hour drive (depending on traffic) and last summer it was 110F. and above for most of the trip. (I saw as high as 115)
That was half a dozen trips ago.