Ok I will jump in here. ( If you don't want to read my supportive thoughts, skip down to the last paragraph)
Synthetic oil is far better than dyno oil. It can work in colder and higher temps so the need for seasonal changes is no longer needed. Synthetic oil also has a higher additive package so it can tolerate longer change intervals. But to be sure as to how much longer, one MUST oil sample and test. Our LMTV's are set up to do this as the engine and transmission have a sample valve built in on the passenger side of the engine near the oil filter.
Now for a light discussion about the oil additives that are in all engine oils whether Dyno or Synthetic. Additives add metal protection and allow the oil to self clean better with the aid of the oil filter. As the engine runs it generates particles and also ingests particles from the air. These are small yet are large enough to cause wear if not filtered out.
Full flow oil filters need to allow all the oil that the pump can move to flow through with little to no obstruction. Hence the term full flow oil filter. This need of high flow means that the oil is not able to filter out small particles so they pass through and cause wear over time. Most full flow oil filters filter out about 20 microns or larger. This means very large particles pass through and will damage the engine. So the additives have a clumping effect taking the smaller particles and clumping them together to make particles bigger and these being bigger than 20 microns can now be filtered out. Over time these clumping additives which act like glue to bind these particles are all used up. It is now time to change the oil and filter. If the filter could filter down to 2 microns or even 1 micron the oil would be so clean that it would cause zero wear. Sadly no full flow oil filter can get oil to this level. We need a different type of oil filter to do this super fine particle filtering.
Enter the bypass oil filter. A bypass oil filter will from day one super filter the oil so that the oil leaving it, is as clean as new oil. Since the new oil starts out clean and it takes time for it to acquire dirty particles this small but ultra high filtration stays ahead of the damage by always replacing dirty oil with super clean oil. Here is where the Synthetic oil becomes an important addition to the team. Since Synthetic oil has a much higher additive package then Dyno oil, it can particle clump much longer thus adding time to the oil change interval. With the added bypass filter it too adds time to the oil change interval. Over the road truckers have known this for years. They change their oil once or twice a year, determined by oil analysis. It is this oil analysis that lets us know how well our engine and it's oil are doing and when it is all worn out and needs to be changed.
As a side point Railroad locomotives only run used engine oil. The old dirty oil is recycled and super filtered to >1 micron and a new additive package is added to bring it back to within spec. It is far cheaper and since it has gone though many miles has stabilized it's viscosity. So 40w is stable 40w. New oil will shear down from 40w to 30w over time.
Now as to oil viscosity weight. A single viscosity weight is great as long as the environment is within the proper working temperature. So if you plan on using your truck at the equator where the temps are within 15 degrees year round fine. Go with 40w oil. However, if where you are using this truck has winter and summer temps and these go from say 15 below zero to 115 above zero in the course of a year, you will either need to change your oil to a lighter viscosity in the winter to a medium viscosity in the spring to a heavier viscosity in the summer and back down in the fall and back again in the winter.
OR you can go with a multi viscosity oil and leave it in all year long with complete peace of mind because you have oil sampling to assure you that your oil is fine.
Most diesel oils are 15W40 this is a great all around weight as it will be 15W in cold temps and thicken up to 40W at higher temps. Protecting the engine as it works harder in warmer weather.
Synthetics will not turn into black or dark brown goo as it heats up. Remember that Mobil 1 oil commercial where the oil is heated in a pan? Synthetics are kings in the heat. This is what is used in hot jet engines. No Dyno oil can take this type of heat it will just gum up and stop moving.
As was mentioned before, there are oil filters that add a additive package to the oil when they are replaced. What this is and how effective it works is beyond this discussion. I am sure it is fine since these are included on MRAPS. To really know what is going on here would require a series of samples over time and comparing the oil with oil that is changed more often. Also one needs to look at a cost VS benefits thing. It might work but cost says it is not better.
So this should give enough to know what you should do.
Here are my final thoughts. If you hate changing oil often and want to save money... Use FULL synthetic 15w40 oil not blended. I use Schaffers 9000 it is about as good as it gets and is cheaper than almost any other brand. Mobil 1 is great but spendy. Amsoil is good but $$$. Consider adding a bypass oil filter. In the past I have used OIlguard. They are now another owner name IIRC. Look at what is out there and try to look through the hype.
I also would only use CAT oil and fuel filters. In my investigation these are the real deal and perform better than almost any other brand. Stay away from WIX and NAPA these are junk IMO. Baldwin is 2nd to Cat and Racor fuel filters are equal to CAT.
