Originally Posted by SHAMUS
I want to change to a 5W-30 as a better warm temp oil (80 degree summers, 40 degree winters) up here in western Washington (Seattle area).
That's mild, really mild.
And in reality, even if ambient or peak was 90, 100, or 120F, it's still low compared to the combustion temps (thousands of degrees), internal engine temps (190-250F), etc.
There's likely a caveat somewhere that discusses going up a grade if heavily loaded. That's a good point - time at temperature is a key element to longevity, and it's the ti,e that your (making up a number) 300hp engine is loaded at 200 hp versus 0-50 hp. Note it only takes around 50-60hp to cruise at 60 mph down the highway.
Why? Because when you're needing 50 hp of propulsion, you're making 100hp of heat. When you're sustained higher loading due to towing or whatnot, then your making 200hp,for propulsion, but making 400hp of heat. Big difference when a decent percentage of the heat goes into the oil.
Having a slightly higher or lower delta t is fairly minimal compared to the sustained loading thst is expected. 40-80 is ideal conditions and the OEM defined viscosity will serve you just fine. If you'll be highly loaded for long periods in the heat, then a grade or two up may well be justified.
