I am currently running 10-40 in my 1996 4.0 OHV. I am wondering what is a good thick oil I can run for engine longevity. I rarely use my truck and when I do use it is to go to the junkyard to pickup parts about 2 miles from my home or to rip around on dirt roads, so I do not care about fuel economy. Would 10w-40 be a good choice of oil ? I live in Central California, and we get a few days a year below freezing where i would have to cold start at around 28 degrees F.
Opinions about oil on the internet are so numerous to the point of absurdity. For the most part, oil is oil - keep it clean, keep it full - and you'll be fine.
That being said, unless you're towing or racing and revving your motor to the moon out in a hot desert, a high viscosity oil in a 4.0 OHV isn't adding any more "protection". As long as you have proper oil level and pressure, the factory 5W-30 or 10W-30 will keep everything happy. If you live on a farm and have bulk 10W-40 that you use for everything else, it's not the end of the world to use in your truck too. Keep in mind though, oil viscosity isn't just about bearings, and thicker isn't always better. A thinner detergent motor oil works better in the 4.0 to flow through the tiny hydraulic lifter passages, and to keep piston oil rings clean.
This is just an anecdote (and I am not a lubricity engineer), but when I recently tore down my '95 OHV with 300,000 miles, I had the following observations.
1. Cam and main bearings were within tolerance, with no scoring or heat damage (if you would have told me 30k of wear instead of 300k, I'd believe you).
2. Rod bearings were only slightly blemised, and slightly out of tolerance.
3. Internal block and head surfaces were all clean and relatively varnish free, with a small amount of sludge in the bottom of the pan.
4. ALL (every single one) of the hydraulic lifters were siezed internally. They floated smoothly in their bores, but the internal plungers were plugged with rusty-varnishy sludge.
5. ALL the pushrods had 1/8-3/16" flat spots on the tips
6. Most of the rocker arms had flat spots at the valve tip.
7. Most of the pistons had stuck oil control rings, and plugged oil drainback passages.
If you never use the truck in freezing temperatures, then just use a name brand, high detergent, conventional (or blend) 10W-30 with an API S tag.
Change oil and filter every 3000-4000 miles. Also replace the PCV valve every couple of years.