Since it appears you're running prime, even with the low load factor, sticking to the recommended maint intervals is probably the best way to go. You're likely causing more rapid oil breakdown to the lube oil since the engine is likely generating more soot and crankcase gases. It's also likey based on where you stated the units are that you are running on fuel with higher sulphur and contaminate levels.
Lube oil sampling would be a help if you can get reasonable turn around from the labs, both wear metals and IR (IR requires a new oil sample to most accurate), should be dome, as well as a TAN or TBN test to determine how well the additive package is holding up in the current service, you can use that info to adjust your oil change intervals if desired. Other maintenance associated with the 250 hour interval, like fuel filters, coolant conditioner checks, and air filters should also be done, even with the lower loads, your environment plays a big factor in how often all that really needs to be done. By adding some instrumentation like fuel filter restriction gauges and oil filter differential pressure gauges you can get an idea of how well these systems are doing their jobs and if you room to move the intervals out.
On overall engine life I believe you'll find it decreased by running extended periods with light loads, I recently was asked to do some failure analysis on two Cummins KT series engines, older units with relatively low hours, but found based on hours and fuel usage records the load factor was quite low, around 15%, the cylinders were badly polished, the top piston land and ring grooves were heavily carbon packed and the rings badly worn. Also noted that most of the valves appeared to be leaking due to heavy deposits in the seat areas. I have had similar experience with many brands of engines, newer engines seem to be more sensitive to load factor, turbocharged engines with keystone ring packs do very poor with light loads as compared to NA engines with rectangular rings. Poor cooling system maintenance and temperature regulators not working properly compound the problems.
A portable load bank on a few hours a week would go a long way in helping the engines meet their target service life in my opinion. At one site we added some large electric hot water heaters and ran them for equipment washing and laundry, look around there might be a relatively painless way to intermittantly increase load once in a while to help burn some of the crud out.
As to Bill's question, I have seen a few different types of calculations done to help extend some maint intervals, either with a calculated fuel consumption or a load or duty factor. Sometimes they seem to help a bit, sometimes the results from oil samples and filter inspections drive a change opposite from what the controls are trying to say you can get away with. Best reults I've seen is in units extending intervals going from 80-100% load factor down to 50-60%, below 50% I have found maintenance usually starts to trend upward toward the full load numbers, and very light loads often cost more in maintenance and repairs than do units operating at high loads, at least in my experience.
Most manufacturers advise maintenance intervals based on "normal" conditions, some don't do a very good job explaining what normal is. Both CAT and Cummins have some pretty good application and maintenance literature, but many factors influence things like oil and filter life, fuel filter change intervals and major services like valve and fuel system adjustments and overhauls.
Monitor fuel and oil consumption rates and trend them, when they start to deviate from the trend things are going bad, the more the deviate from the trend the worse things are getting. Oil smaple reports work the same way. I am also big on opening up a used oil filter and peeking inside, can tell you alot about the health of an engine.
Hope that helps,
