Would 40 weight work in summer temps of 100 degrees? Kansas City's summers can get hot every now and them. If I have trouble getting 50 I might go with 40.
I am pretty sure hotter higher wt, colder lower wt. is a basic rule to follow in all oils, but there are exceptions, like age and wear. so really U are thinking the opposite,
Treeguy , yes the 80/90 wt is fine for your truck, its what has been used in service since it was new probably, for sure in the guard where most of these trucks spent most or all of there lives, but like said in the tm it states 50 wt in the trans and transfer, hey its the way to go for alot of guys, but I would bet in severe service the 80/90 is better. thats all i run, thats what is in em when we get em. for the engine in todays oils 15/40 is the best in almost all climates, a multi temp oil changes in temp to offer the best all around protection , and mostly for start up when it is critical, oil needs to flow in the tight ares fast and a heavy oil will take critical time to get there, a simple rule of motor oil is, a tight new motor a lighter oil, an older (looser) or more worn motor a higher number or thicker oil, basic rules of thumb for oil. oils can make one crazy they make one for everything, most of that is marketing, if U want the best answer, call the 800 number on the oil can u are thinking of using, thats where i have gotten all my info during the past 30 plus years, the techs there are wayyyy better than any mechanic or guy on the street. oh and i have found it is worth it to ask the same questions to different manufacturer techs and when they all say the same thing U know U are onto it, thats were my info accumulates from good luck.
