Oil is basically simple, the lower numbers are for newer tighter tollerence motors, the 15 /40's are more for diesels, they are just fine for the climates U will find in conus...or continental United States...so the lighter the oil the faster it will lubricate tighter tolerences, and the heavier oils are for bigger more rugged motors, but then again temp plays an important role as U have mentioned, it can get so cold that u could need a 30 weight , but remember the military ran straight 30 in these trucks for most of there life, as time and oil got better, they went to an oil that covers the most ground, and that is the 15/40. all the oil U will ever need, and remeber , any oil is better than no oil, and ALL oil mixes. do not believe old wives tales, the better the oil the better the lubrication, It is wise to warm the block of any vehicle if U want the least amount of wear. Oh and an old worn motor can last a few more years with straight 50 as long as U watch the start up temps...and so on, read the label and use common sense,
