What yr and make / fuel / engine are these vehicles? Do they run hot enough to not condensate?
For commercial use I often go 20,000 miles on synthetic on rough service I change filter every 5k (I have used many, but really prefer Amsoil and buy it cheap with a commercial discount) It really does test superior to mobil I
+ it is great in gearboxes and trannies.
(I have over 3 million miles on fleet, and most of my current fleet is over 300k per each vehicle) I have never had an engine failure due to oil issues.
Run an oil analysis every 5 thousand miles until 25,000 miles on each rig, or stop and change when you note a degradation of properties (only do the test once, unless something changes)
You can add Bypass filters that can greatly help keep oil fresh, but if there is nothing wrong with your engines, and filters, & fuel is clean they can run A LONG time between changes. I fact many over-the-road truck use a metering device to continually run used motor oil into fuel system, and replace with fresh oil.
An automotive forum is a very polarized environment to get OIL advice that works for all, BUT... an informed fleet manager will be able to advise you. It does really depend on the type of use and engines. I run a few of mine on Dino, as they don't like synthetic (tummy ache?). They just prefer to run on Synthetic (One motorcycle INSISTS on dino, tho I would much prefer synthetic). HEAT is the worst killer of lubricity, you can add an oil cooler for about $100 and it will probably prolong changes even past 20k in a 'road' vehicle.
Do make sure you fuel these vehicles in the evening to avoid condensation overnight.
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