Do you run an engine before you shut it off and check the oil?
Very rough rule of thumb for 4-stroke gasoline outboard motors is that 100hrs = 10,000Mi in a car. Assuming you're running irrigation pump engines, they're probably not running @ 4k-6k RPM though.
If I had a motor that HAD to work remote on long intervals I'd probably add a 2nd oil filter and REALLY good air filtration.
