Sunchaser,
I first became curious about synthetic oil and change interval when I found the Rotella T6 still appeared clean in my Yanmar at 150 hours. I had never seen this in any diesel engine before. I assumed it was due to quality oil, good filtration and a clean, modern engine. Since the oil appeared so clean, I began to wonder if it would be safe to run longer since the synthetic oil manufacturers use terms like 'extended life' etc. in their advertizing. Remember, I have
not had any oil analysis performed so far. My only clue to the oil condition was the look and smell when it was changed, which obviously is not a very reliable way to gauge the condition. That is precisely why I am going to begin having the oil analyzed. If the chemistry is good, I am only reaching for 300 hours max.
As for the thousands of hours between diesel oil changes, there seems to be at least some anecdotal evidence to support the claims:
1. The Louisiana Department of Natural Resources published a report entitled
90,000 Mile Diesel Engine Oil Changes where they state "Sixty-thousand (60,000) mile diesel oil change intervals have been successfully achieved in fleet operation using Amsoil synthetic based oils. Amsoil has produced premium, synthetic based API approved diesel engine oils since 1972. Present fleet tests of 90,000 mile drain intervals have found the oil free and clean of deposits and suitable for continued use. Amsoil claims a four to eight percent increase in fuel economy when using their product."
2. An article about Amsoil Synthetic - 409,000 Mile Oil Change Interval
409,000-Mile Oil Change Interval with AMSOIL Synthetic 15w40 Diesel Oil
3. I also found this comment on a trucker's site regarding synthetics "Top quality synthetics like Delvac 1 are capable of going 100k in the right application. To be totally safe, the best thing to do is run it for the normal interval (6 months or 6k) and send a sample in for analysis. The lab can give you a good guesstimate on how much longer it could have safely run. Run it that far the next time and retest. A filter change can make a big difference in how long oil can be run safely, the quart of make-up oil gives a "second wind" to the TBN."
If true, these accounts do equate to oil change intervals of over a thousand hours, but definitely not 'many' thousands. Oil analysis and frequent filter changes seem to be the key elements in determining the optimum extended oil change interval.
Finally, I would always agree the safest approach is to follow the manufacturer's recommendations. In my case, the only nuance is the introduction of full synthetics into the conversation. They are not mentioned in any of my Yanmar service manuals or literature, all of which was printed before they came into common usage.
Thanks for your input.
Larry