The 100 hr. interval is dictated strictly by oil analysis. I don't like to cut it really close but 100 hrs. gives a really good clean analysis. It's really the best way to know the health and longevity of an engine that gets beat pretty hard.
How do you do an oil analasys? What does that cost?[/QUOTE]
NAPA has a kit, you mail it in with $20 and get a full report back. Also google oil analysis for other companys.