Hello all,
This is an old thread but still important! ()
I have a F-250 1967 and after reading many boards and researched what oil types are out there I personally think the winter weight should not go below 10 for most areas in theses older trucks. 5W is fine if you drive your older vehicle in the winter and it is good to keep the oil from gumming up in colder weather (when starting the engine in below zero temps). The high number does the opposite and helps the oil from getting too thin as the engine really heats up. With the older vehicles you should not go below 30 ever! 40 weight is a great standard since these engines can get toasty especially in the areas that are consistently in the high 90-100 degree Fahrenheit ambient temperatures. A 5 quart jug of the High Mileage additive in the 10W40 will run you around $17-18.