I would have to agree with bruce381. I doubt changing oil is going to cause a drastic change in your operating temp's. Your problem is a cooling problem more than it is a lubricating problem. Will high quality lubricants lower temps? Sure, but not enough to take a real notice of in engine temp.
First thing I would check is if your fan clutch is operating correctly. If you have a viscus fan clutch it may be bad and not locking up the fan. This means it is not spinning as fast as it should and decreases the air flow across the radiator. You may find if you have a electric or oil operated fan clutch that all you need to do is change your temp sensor in the fan clutch system to a lower temp. This would cause the fan to turn on earlier before the temps get to high. Next, check and see if it is possible to install a lar*** diameter fan on the engine. You may also find that you can get a fan blade with more blades. This also increases air flow across the radiator.
Another thing to check is if the Air to Air cooler and the radiator are pluged with dirt. This is a very common problem in off road trucks. I have repaired alot of trucks over the years that had high operating temps and found the cause to be dirt trapped in between the cooling fins of the radiator and Air to Air after cooler. You might just be supprised how much crud get's stuck in between the fins and how much it effects the performance of the radiator. Try a air blow gun and blowing it out from the back side. If you see large clouds of dust you found your problem.
Still no luck go with the addition of a electric cooling fan as mentioned previously. Anything to increase air flow across the radiator.
How old are the trucks? If your filling your radiators with tap water and not distilled water you might have a pluged radiator. The internal pasages in the radiator collect the hard water particles and over time restrict flow or completly block it. It's another possiblility.
Good high quality lubricants in the rear axles, transmission, transfer case (if they are 6x6 or 8x8 trucks) and engine oil are the start of any good preventative maintenance program and can greatly reduce maintenance cost and down-time. The problem is the cooling system and the charging/battery system are very often overlooked if not forgoten altogether until problems arise.
If you have transmission and rear axle temps that are high you can often greatly reduce their operating temps with a good quality lubricant. It's another place to check for coolers to. Some transmissions and rear axles have external coolers that are hidden under the truck and collect high amounts of dirt. Be sure to keep them clean as well. I have found Texas Refinery Corp.'s gear/transmission oils are great at reducing operating temps. I have personally worked on some off road loging trucks that had their trans and rear axle temps reduced by as much as 60-70 dregres by just filling them with a superior lubricant.