A simple an inexpensive oil analysis is worth it. If nothing else, it feels good to read the analysis that tells you how good your oil and engine are doing. It's only $25.00 from Blackstone. Yes, a history of oil samples will tell a story, but for the most part you will get a report that says everything looks good. If everything is good then you are reassured and there is really no need for a followup analysis for many thousands of miles. A series of test is usually called for if the analysis finds an area out of the normal range. In that case you might be taking corrective ation and want to see the results.
Besides all the technical data, a real person write comments for each sample. I suspected the injectors on my Duramax were on the way out. I sent in a sample to confirm my diagnosis. On one sample the fuel made up 2% of the oil. That was just outside the acceptable range which is "less than 2%. Another sample right before I installed new injectors had the fuel composing 3.5% of the oil with a significant drop in viscosity. After I installed the new injectors I sent in a sample to confirm I had solved the problem of fuel contamination. Here is the reassuring personal comments that accompanied all the statistical data:
EDWARD: The fuel that had been plaguing this engine for last two samples finally disappeared here. Wear
metals continue to read nice and low even though this oil was in use almost twice as long as last time. With
wear this good, you might as well go longer on the oil, but we wouldn't blame you if you just wanted to stick
with what's working. The viscosity was normal for 15W/40 and low insolubles and silicon show good oil and
air filtration. Nice engine as of sample date 10/1/11. Keep up the good work.
$25 well spent for peace of mind.