A lot of folks running diesels rely on Blackstone Labs or another testing facility to determine when their oil needs to be changed. Same way in the marine industry.
They change filters, and every 10,000 miles send a sample off to Blackstone. I believe the analysis costs about $11. Some people go 40-80k miles before they accumulate enough acids etc to require an oil change. When you're changing out 35-70qts of oil, it makes sense.
For my gass...just not worth it. GM says to go by the oil change computer in my cars that are equipped with one. One car had a change in as little as 7,500, and as much as 12,000 depending on how it had been driven. That same engine in a motorhome, according to the documentation, should be changed every 3,000?
Why do you think that is? :)
Jim